Please read your text and watch my video. What frequencies are seen on the display then, if they are not piano's... ? You're not hearing a difference because you're cutting a frequency range where there is little or no signal from the piano -- notice that the spectrum analyzer display that you turned on shows which frequencies the piano sound is made up of, and it shows that the piano sound is mostly above the frequency you chose to cut, so there's nothing there to be affected by the EQ. https://youtu.be/WrZc9nL2_cg

The EQ display shows all frequencies, not just the piano's frequency range. The piano's frequency range is shown by the moving curve -- the spectrum analyzer display. You chose to make a narrow cut to a particular frequency range below the frequency range of the piano, as shown by that moving curve -- there's no audio down there, so your EQ cut of course has no audible effect. Also, a very narrow cut like that may not have an obvious effect on tone even if it were applied within the piano's frequency range [which it is not]. If you want to train your ears and learn about the effect of EQ, try larger, wider boosts [and cuts] at different center frequencies within the range of the audio you're applying the EQ to [as show by that moving curve spectrum analyzer] -- then you should be able to hear tonal changes caused by the EQ, and you can start to get a sense of how EQ can affect the tone of an audio signal.

I enclose again the same video. About frequencies I understood now, but if you listen to the video, you are listening the loud click in several times. What is it? Why it happens? How do I get rid of that? Does it will be heard on the recording, too? https://youtu.be/WrZc9nL2_cg

The clicks do not appear to be coming from the track playing in GarageBand -- the meter on the track does not react when the clicks are heard -- plus clicks are heard at the beginning when the GB track is not even playing, The source must be something else -- if they're coming from the computer speaker, maybe check that nothing else is running in the background..?

I checked out carefully. The clicks coming, when I put the mouse cursor to the Q numerical display, a click, and then I am changing numbers of Q. Is it normal? If you change your Q, do you also have clicks before of that.. ?

No, you should not hear clicks when clicking on or adjusting Q, that's not normal. Is that click sound coming through the computer speakers or is it just the video recording picking up the sound of the mouse or trackpad being clicked in the room [that's what it sounds like to me]? Do you only hear clicks when you click on the Q control, or do you also hear clicks when you click on other controls? If you get clicks any time you click on anything, and if you're using a trackpad, apparently some Mac laptops have a fake click sound that might have gotten turned on and might be able to be turned off in System Preferences > Trackpad > Silent Clicking..? https://support.apple.com/en-us/HT204352 I don't have a laptop or a trackpad so I can't offer you any more advice than that about it.. On the other hand if you only hear clicks on the Q control then I have no idea what's causing it.

Yes, you are right, it's the trackpad, I checked out. All controls have that click. I also checked out system preferences-trackpad, but I don't have options silent click and force click, as shown on the bottom of the apple support page you enclosed to me. Why I don't have? What to do? How EQ works at all? Firstly I choose Q and then use EQ, right? It means, click will be not recorded to my song.. ? Or do normally people change Q while they are boosting and cutting, or they are setting up Q first, then don't touch it anymore..? Video is enclosed. https://youtu.be/EGekHwuP5OQ

Apparently some laptops and some versions of the OS don't have the option for Silent Clicking -- I guess yours is one of them. If that's the case then I suppose you're stuck with it. Fortunately that click sound should not be part of the audio in GarageBand, so it shouldn't get recorded. But if it bothers you I guess you could buy a mouse and use it instead of the trackpad. You'd normally dial up all the settings for the EQ you want on a track and then leave it, so once you've found the EQ you want you'd just leave it at those settings -- there should be no need to keep playing with it.

You say I normally dial up all the settings for EQ. But then you continue the sentence "you want on a track". Does it mean, I need to dial up the settings for each track separately? Or do the settings will be all the same during the whole project? Even all the projects maybe... ?

There is a separate EQ for each track, plus an overall EQ that affects all tracks. In the Smart Controls > EQ panel at the bottom: Select the EQ display: To use the individual track EQs, make sure the Track tab is selected; Select a track in the track lanes above; The EQ you see/adjust will now only affect that selected track. Select a different track -- you'll see the EQ setting change: its a different EQ, which will affect only the currently selected track. You can set different EQ settings for each track, so you can get the tone just right for every track. To use the overall EQ that affects all the tracks, select the Master tab (just to the right of the Tracks tab); Now the EQ you see/adjust will affect all tracks. Use that Master EQ if you want to change the tone of the entire song. To go back to EQing individual tracks, select the Tracks tab again and select individual tracks with their individual EQs. When you use [any] EQ the settings you dial up will be in effect for the entire Project/song. All EQ settings [individual track EQs and Master EQ] will be saved with the Project/song, and they only apply to that particular Project/song.