Microphone audio is the sound that your computer's built-in microphone (or any external microphone you're using) captures during the recording. Enable microphone audio if you will be narrating over your recording.
Only record your voice with microphone audio. If you try to record the sound coming from your computer's speakers with your microphone, the quality will be very low.
Tab audio is any sound that occurs from within your browser tab any plays on your computer's speakers. For example, if you play a video on YouTube, the sound coming from the video is tab audio. Tab audio is only available when you record your browser tab.
If you want to record tab audio AND narrate over your recording, we recommend using headphones. If you don't use headphones, the tab audio coming from your computer's speakers may be picked up by your microphone and distort the sound.
System audio is the same as tab audio, except it refers to all the audio originating from any of your computer's programs (not just a browser tab). Capturing system audio is currently available for desktop recordings on Windows machines and Chromebooks.