In FireCut, Multi-track means to edit your video by considering multiple tracks separately (conversely, all other features will treat the audio in your timeline / selected tracks as one single mixed-down track).
It offers 2 main features:
Multi-track video editing to edit podcast-style videos automatically, by switching between multiple video tracks based on multiple audio tracks. FireCut will "follow" the loudest audio (similar to how video conferencing tools like Zoom and MS Teams will show the speaker on the screen), plus add some variations in the final cut to prevent monotony
Multi-track audio muting to disable quiet audio in selected tracks to remove background noise, by listening to each track separately. This is similar to the silence cutting feature, except it analyses each audio track separately (rather than a combined mix-down) and only disables / mutes clips rather than deleting them