You may have heard this term before, although maybe not in reference to literature.
In literature, the term refers to a style of narration in which a character's thoughts are thrown at us, one after the other, as if we were thinking those thoughts right along with them.
Pioneered by Modernist writers like Virginia Woolf, this narration gives the illusion that we're privy to a character's unedited, uncensored, and sometimes more than a little nonsensical thoughts. After all, our own thoughts don't always make perfect sense, so why should a character's? The idea here is to represent as accurately as possible human consciousness. Talk about a tall order.