In the production process of any kind of animation (2D or 3D) the story is broken down into chunks called sequence. Each scene consists of smaller chunks called scenes.
There are multiple people working on a scene/sequence in a production pipeline, so it becomes necessary to have everyone follow a common set of instructions.
Exposure sheet/ X-sheet is a tool used in animation which contains instructions to be followed for a particular scene. It contains notes about the action, timing, camera movement and additional footnotes as required.
In old studio environment, x-sheet also served as an indispensable communication tool between the studio and the cameraman who’d shoot the final cels into film.
Since this helps the animator to create a map of the animation for others to follow, it is also popularly called a Cheat Sheet or Dope Sheet.
You can act out a scene physically before you start doing the thumbnails, you can scrub through the audio and time out exactly at which frame each vowel sound happens, and you can time out the actions the character makes according to the audio.
But once all of this is laid out on an x-sheet, it becomes much easier to manage all of this and keep all those things in perfect synchronization.