HMD stands for "How's My Driving"; it's a post to leave constructive criticism about a certain character's portrayal that can be found usually on a character's journal or on a muse list post of the mun.It is common courtesy to have an HMD post on a character's journal, preferrably one that allows anonymous comments and does not show the commenter's IP address in order to make the critic feel more comfortable about posting their thoughts.
Additionally, it is crucial that when leaving an HMD for certain kinds of players, you leave at least one inflammatory anonymous comment. Good examples of these people include (but are not limited to):
The eventual outcome for giving these players critique will result in barrels of lulz. Many times (if not always), the player will respond with something heinously tldr, disable comments in their HMD post, delete their HMD post, flounce, write a PSA and post it on their community of choice, or otherwise cause massive secondhand embarrassment. Fellow players will respond with heartfelt farewells even if they genuinely don't give a shit.