Valve

The three valves, keys, on a trumpet add lengths of the tubing to the instrument to lower the pitch. The first valve lowers it by a whole tone, the second by a semitone, and the third by a minor third. By adding tube length it lengthens the distance the air is traveled, lowers the frequency, and alters the pitch. When no valves are pressed it has a tube length of 148 cm, pressing valve 1 adds 17.9 cm, valve 2, 8.6 cm, and valve 3, 27.8 cm. With the combinations 0, 1, 2, 3, 12, 13, 23, and 123 and altering your lip oscillations you are able to play the chromatic notes through multiple octaves.
They also have a condensed trumpet called a "pocket trumpet" that has the same length of tubes, valves, and mouth piece but is squished together. I own one and they sound the same as a regular trumpet and are fun to play.