Slots are the building blocks for revisions: MediaWiki core and extensions define a certain number of roles that a piece of content can play, and a content associated with a certain revision in a certain role is referred to as a slot of that revision.
(For example, a template page could have a
main slot holding the code of the template, and a
documentation slot holding the documentation.)
Slots have been introduced in MediaWiki 1.32 as part of Multi-Content Revisions. At this point in time, most interfaces understand them, but fall back to using the
main slot for backward compatibility (which is the same thing as the content of the revision pre-1.32).
Slots are stored in the slots table.