Someone’s bark is worse than their bite, or all bark and no bite.

This expression means that although someone says things that sound frightening, the person’s actions will not be as severe as the things that were said:

For example:

The boss seems mean, but his bark is worse than his bite.

Mr. Martinez seems like a hard teacher, but his bark is worse than his bite.  He will always be there to help you with your work.

The basketball coach has a really loud voice and yells a lot, but really, he’s all bark and no bite.

I know he seems like he will be hard to make a deal with, but really, the lawyer is all bark and no bite.

The substitute teacher used to be a Marine Drill Sergeant, but his bark is worse than his bite, and I know you will like him once you get to know him.

Tradução:

All bark and no bite refere-se a uma pessoa que parece agressiva ou perigosa, mas não tem ação ou força para realmente causar dano.  Cão que ladra/late não morde.