All musical instruments make a sound under the control of a musician. In other words, at any time, the musician can cause an instrument to start making a sound. For example, a musician can push down a key on a piano to start a sound. Or, he can begin dragging a bow across a violin string to start a sound. Or he can fret and pick a guitar string to start a sound.
Most instruments also allow the musician to stop the sound at any given time.
For example, the musician can release that piano key, thus stopping the sound. Or, he can stop dragging a bow across the violin string. Or he can release his finger from the guitar fret. This stops the musical note from sounding. There are other things that many musical instruments may have in common, for example, most instruments can make sounds at various volumes.