It's a phrase used to tell someone to relax, stop being aggressive, or to be less assertive.
Here are some examples of how you might use "baja la mano" in English:
* "Calm down, baja la mano! There's no need to get so worked up."
* "Lower your hand, baja la mano. I'm not trying to start a fight."
* "Baja la mano, it's just a game. Don't get so serious."