Proteins are the basic building blocks for a large part of our body. It makes up our organs, muscles, bones, and many of the chemicals necessary to sustain life. It's found throughout the body. It is made up of Amino Acids that are combined in thousands of ways to create the proteins necessary to repair and replace cells that die during the normal course of our lives.
Protein is most commonly associated with muscles because you need a bunch of protein to build muscles.
Our bodies cannot store amino acids, so we need to either make them from molecules through chemical reactions, or modify existing amino acids to create the needed amino acids. Because we can't store them, we need to get the building blocks from continually eating food.
There are nine amino acids that cannot be created by our body, so we must eat those. These are known as essential amino acids.
The National Academy of Medicine recommends a minimum of 0.8g of protein per kilogram of body weight.[1] This is a bare minimum. More recent research shows that up to 2.2kg per kg of body weight is tolerable for most people.[2] For athletes, it can go even higher.
For me, at 155lbs (~70g BW), I eat between 120-180gs per day depending on the day. I skew towards the lower end on rest days, and the higher end on lifting days.