Compound exercises recruit multiple muscle groups and two or more joints at once. That is opposed to isolation exercises, which target one muscle group (like bicep curls). Because they help you get more done in less time, they’re great for increasing overall muscle mass, and they also burn more calories because they require more energy output. Compound exercises can be single moves that put multiple groups to work at the same time (like lunges and squats), or they can be two moves strung together (like bicep curls to shoulder presses).
To make the most of the time you put into the gym, you should aim for compound moves to take up 70 to 80 percent of your workout (and target specific muscles you want to work with isolation exercises the rest of the time).