DM stands for Direct Message, a feature that allows users to privately message each other on social media platforms. It is a way to communicate with someone without the message being visible to the public.
For example, on Twitter, you can send a DM to a follower to ask them a question or share something personal [follower: a person who subscribes to your account].
Very Simple Definition:
DM means sending a private message to someone on social media.
For example, you can send a DM to a friend to ask them how they are doing.