Modern DAWs and virtual instruments make recording and creating orchestral parts much easier than it used to be but while they can provide the tools, they can't write the music for you. An understanding of the specific skills and techniques required to compose for orchestral instruments - be they real or programmed in MIDI - is essential. There are playing styles and terms that are pertinent to certain orchestral instruments - glissando and portamento being just two of them. Luckily, help in understanding these is at hand!
In this short video from the course Orchestration 101: The String Section, Thomas Goss takes you step by step through what glissando and portamento techniques are, and why they are important to orchestration and scoring.
Using live action and written scores as examples, Thomas explains how certain playing techniques translate to physical movements and hand positions on stringed instruments. Using live playing, he shows how these techniques allow much more natural and flowing performances when employed correctly. Be sure to check out the rest of the videos in this course for a complete guide to orchestrating live strings!