For the Wikipedia essay, see WP:Too long; didn't read.
TL;DR, short for "Too long; didn't read", is an expression used in writing to say that some text being replied to has been ignored. It originates in Internet slang, where it is often used in quoted responses to indicate parts of the text skipped as too lengthy.
The abbreviation is based on the principle that, if the writer does not invest the time to convey their message concisely, the reader is justified not investing the time to read it. Alternately, I might mean that there is insufficient material of value or interest to justify the time required to read it.