What 'Sides' Are
The pages you actually get, rather than the whole script.
Sides are the specific pages you are given for an audition or a scene, rather than the entire script. When casting sends "the sides," they mean the handful of pages, sometimes one scene, sometimes two, that they want you to prepare. You may never see the rest of the script. The sides are usually all you get, and all you need.
The word is old theater shorthand. Before scripts were cheaply copied, an actor was often handed only their own part, with the cue lines that came just before each of their speeches and nothing else. Those partial pages were the actor's "side." The full text stayed with the stage manager. The practice faded, but the word stayed, and today it covers any subset of a script pulled for a specific purpose: an audition side, a callback side, a scene for class.
In practice, sides are how most auditions actually arrive. You get an email with a character breakdown and a few pages attached, sometimes the night before, sometimes an hour before. The pages might drop you into the middle of a story you know nothing about, with references to events you never see and a relationship you have to infer from a single page. Reading the sides for everything they imply, and not just for your lines, is most of the preparation.
An example. You are sent three pages: a tense conversation between your character and a stranger in a parked car. There is no scene before it and no scene after, just those three pages. The sides are your whole world for this audition. You read them for who these two people are to each other, what just happened to land them in that car, and what your character is trying to get before the page runs out. Everything you build comes from what those pages give you, because they are the only thing casting handed you, and the only thing they will judge.
A quick note on usage, since it marks you as someone who knows the room: actors say "sides," not "the script," for audition pages. The script is the whole thing; the sides are the pieces in your hand. For how to turn a set of audition sides into specific, playable choices, see the auditions lane; for the full prep sequence when those sides are headed for a self-tape, see the self-tape lane.