The Klingon Language name for the Klingon Language. The tlh is a voiceless lateral affricate, which means it's kind of like saying a t immediately followed by a voiceless l (or more accuately, a Welsh ll, if that helps). The I (it's a capital i, not a lowercase L) after that is the i-sound in "bit". The ng is like in "singer", and the H is a voiceless velar fricative--basically, like a Welsh or German ch (ach-laut).
Literally, it means "Klingon('s) Language." Pretty straightforward.