What are sides in a film?

Based on my experience, "sides" in a film, TV, or theatre production refers to the audition process. The actor auditioning is given excerpts from the script to learn upon arriving at the audition site. These pages of the script are known as "sides."

SIDES on a film set refer to the pages that will be shot that day. The AD Department will photocopy the pages from the script that are indicated on the CALLSHEET and those pages will be shrunk down to a physically smaller size (from 8 1/2" x 11" to roughly 8" x 6").

Those pages are stapled together and then handed out to cast and crew in the morning so that everyone on set is aware of how the scene reads without having to constantly refer to the full script that is too unwieldy to handle all day. The sides are small enough that they can be shoved into a pocket or hidden away on set so the Actor can have them to refer to until the last minute.