Audio tracks are tracks in which you can add sound clips. Each sound clip in an audio track is cued at a specific time in your animatic, so that it can be synchronized with the action.
Your project can have as many audio tracks as you want. Having several audio tracks is required if you want to play several sound clips at the same time, as a single audio track can have several sound clips cued across the animatic, but can only play one sound clip at any given time. You can also use different audio tracks to organize your sound clips, for example by having a specific track for each character's dialogue, one for music, and one for sound effects. Each audio track can be muted, and you can also enable solo mode on an audio track if you only want to hear that audio track.