An animatic is the first stage of the edit in an animation. It's the combination of the storyboards, audio (possibly with temp voice acting), and timing. It's used as the base for the animation: animators will take the shot angle, durations, certain actions, and timings as a starting point for their work on each shot.
Animatics are also sometimes used in live-action films, in the same way as directors will storyboard certain sequences, and they may be the stage used prior to previz for VFX work on large action films.