It means to accidentally do something that helps the other side and hurts your own position.

For example, if someone wants to provoke you into reacting badly, and you overreact, you may be playing into their hands because you gave them exactly the advantage they wanted.

Simple meaning

  • You help your opponent without meaning to.
  • You make the situation easier for them.
  • You often lose an advantage by reacting the wrong way.

Example

  • “If you argue with them in public, you’re playing into their hands.”
  • “By getting angry, he played right into the reporter’s hands.”

Plain-English paraphrase

You could think of it as: “You’re giving them what they want.”

TL;DR: “Play into somebody’s hands” means to unintentionally help them, usually by making a mistake or reacting in a way they expected.