Enjambment is when a linguistic unit (phrase or sentence) in poetry runs over the line break. Enjambment is the opposite of end stopping, where each linguistic unit corresponds with the line length.
For example these lines from T.S. Eliot's poem "Gerontion" are heavily enjambed:
"After such knowledge, what forgiveness? Think now
History has many cunning passages, contrived corridors
And issues, deceives with whispering ambitions"
Meaning flows from line to line, and the reader's eye is pulled forward. Enjambment moves the poem forward, it accelerates. Compare the enjambed Eliot with these lines, from Alexander Pope's "An Essay on Criticism," which are completely end stopped:
"Nature to all things fix'd the Limits fit,
And wisely curb'd proud Man's pretending Wit:"
Each line is formally correspondent with a unit of thought - in this case, a clause of a sentence.