Brass instruments such as the Trumpet and French Horn have three keyed/levered valves. Some related brass instruments such as the Piccolo Trumpet have four keyed valves. The Tuba has four or more.
Pressing down a key opens a valves, which adds extra length of tubing to the air column inside the instrument, thus lowering the pitch.
Roughly speaking, the first valve lowers the pitch by two semitones, the second valve lowers the pitch by one semitone, and the third valve lowers the pitch by three semitones. When present the fourth valve lowers the pitch by about five semitones.
Why are the valves values in this order: 2,1,3,(5) semitones;
and not what might seem more logical: 1,2,3,(5) semitones?