When you warm up, you get ready to do something, like run around or sing a few notes before a game or a concert. To warm up is to literally make something warmer, so you can also warm up your soup.
You can warm up beside a cozy fire after skiing, or warm up in your chilly house with a mug of hot tea. Or, warm up your car by running the engine for a while before you drive. If you’re shy, it might take you a little while to warm up to people. A band warms up by playing parts of songs and tuning their instruments, and an athlete might warm up with an easy jog and some stretching.