When you work out, you break down the muscles. The hope is that they will rebuild themselves a little thicker, and stronger. But this is far from guesswork.
Protein is needed to help the muscles recover.
When protein is ingested and broken down, it is broken down into amino acids. These are what your muscles use to recuperate. There are many different possible sources of high quality protein. Then include, but are not limited to whey, chicken, tuna, egg white, beef, turkey, other fish, ostrich, soy. The timing of when the body uses and therefore needs protein, is largely unknown, and different for different people.