Mlem is DoggoLingo for the act (and the sound) of licking, lapping, or just generally having your tongue out and about, especially in a slobbery fashion. In human terms this is probably most closely related to a slurp, but it would not refer to the act of a human drinking, for example, a milkshake; it is specifically the sound a tongue makes when entering and exiting the mouth. It includes licking one's lips, licking one's nose, licking another's nose, and any act of intentionally using one's tongue outside of one's mouth. This is an important concept in doggo culture.
This is companion to the term blop, which refers to the tongue resting outside the mouth, but not doing anything. A note on usage: doggos blop, cats blep, and other animals may blup; however, mleming is done by all cute animals regardless of species.