System audio: Refers to all the sound originating from any of your computer's programs, such as YouTube audio and other people's voices in an online meeting.
Microphone audio: Refers to the sound that your computer's built-in microphone (or any external microphone you're using) captures during the recording, such as your voice.
Tab sound: Refers to the sound generated by This Tab only.
Related instruction(s):
How to record system audio/program sound/tab sound