In a traditional TTRPG table dynamic the Player Characters have the role of the protagonists in the story.
The Game Master/DM has the role of a narrator and facilitator who guides the players and PCs through the game. In order to do that they embody various Non Player Characters (NPCs).
A DMPC is a character that is controlled by the DM but instead of being supporting cast character to the protagonists (PCs) the DM positions them as a protagonist, the focus of the story.
A DMPC does not necessarily have to be a sign of bad DM practices, but it often is. Campaigns with DMPCs run into various issues:
- DMs have much greater focus over the game than other players, which can lead to them favoring their "pet character" over others
- A DMPC can be a sign of a DM who became a DM "against their will" (for example nobody was willing to do it and the game would not happen without it). However, they really don't have any interest in DMing and just want to experience the game as a player.
- A DMPC means that the DM has a player agency in the game they are running which can easily lead to them railroading the rest of the group to what their DMPC "want's to do"