Feedback / Echo
Feedback (反馈) is when there is too much sound, usually because your microphone can hear your speakers/headphones, creating a cycle of increasingly loud, high-pitched noise.
Echo (回响/回声/回音) is when you can hear your own voice a short time after you speak, and is also caused by your microphone hearing the sound from your speakers/headphones.
How to Avoid Feedback/Echo
The most important thing is to have the right equipment.
For listening to a class, use (closed) headphones or earphones, not speakers.
For speaking, using a microphone that is part of a headset is probably the best. A desktop microphone is so-so (but you must be careful how you position it and with setting the volume levels). Built-in microphones (like in the screen of a laptop) are vulnerable to feedback.