Complex sentence: The phrase too long; didn't read (abbreviated tl; dr or simply tldr) has long been used on the Internet as a reply to an overlong statement indicating the reader did not actually read it due to its excessive length.
Split into simpler sentences: The phrase too long; didn't read (abbreviated tl; dr or simply tldr) has long been used on the Internet as a reply to an excessively long statement. It indicates that the reader did not actually read the statement due to its excessive length.