Direct translation would be "lick me". However, it is commonly used as a short form of "Leck' mich am Arsch" (lick my arse) which is rather dependent on the situation. It can be used as an expression of amazement, as a curse or rather casual if you don't want to follow a request from someone who'd better be a friend ("Can you give me a hand with getting those 500 pounds of sand over to the other pile?" "Leck' mich!")
Using it When having sex in Germany could be considered rude, so watch your tongue when you're outside an S&M club.