A tuning hammer is the tool used to change the tension on the strings of a piano. It's called a hammer mainly because of its shape. It's a metal rod with a handle. The head of the rod is bent 90 degrees and has a socket wrench-style end. You can get one for around $30 at any good piano dealer.
An important note: vice grips or pliers are not a good substitute for tuning hammers. You'll strip the pins on your piano and, after seeing the damage you've done, no self-respecting piano tuner will ever visit your piano again.