When the term "band" is used in relation to EQ, it means a range of frequencies. An EQ consists of several controls, each of which boosts or cut a specific range of frequencies [within the audible frequency range of 20Hz-20kHz]. Each individual control, acting within its own frequency range, is a band. So an EQ with 4 separate controls for boosting or cutting 4 different frequency ranges would be a 4-band EQ.
You can watch the videos on EQ in this course:
Or for a more detailed explanation, you could watch videos 2,3,&4 in this course [it shows the EQ in Logic, but its mostly the same EQ as the one in GarageBand]:
Or for a more detailed explanation of EQ you could watch videos 8,9,10,11,12,&13 in this course [it shows different EQs, but the basic concepts apply to all EQs]:
But the best way to learn about EQ is to practice with it yourself. If you go to the Smart Controls panel in GarageBand and click on the EQ button, you'll see an 8-band EQ -- you can experiment with boosting and cutting at different frequencies to hear what it can do. If you have a recording that you think has too much tone or not enough tone at a certain frequency, you can change the tone with the EQ. Ultimately, you'll have to experiment with EQ a lot to train your ears to get familiar with it, and to learn when and how to use it effectively.