A "Dad Move" is a way for DMs to deal with problem players.
Let's say you are running a game with a kid playing. He's your friend's son, and his dad is excited to share his hobby with his kid.
The thing is, this kid attempts to kill everyone, makes terrible choices on purpose, etc. You can't blame him—he's just a kid who wants to have some fun. However, he's a pain in the neck as a DM and really sucking the enjoyment out of the game for the other players.
So, you pull him aside after one of your sessions. Putting on your best "Dad Voice" (or "Mom Voice") you explain to him that although he should enjoy himself, he needs to respect other players, etc.
To summarize, a "Dad Move" is treating him like you would your own kid, and kindly—but firmly—explaining that the player needs to stop their problematic behavior.