When a sound source moves toward a listener, the observed pitch generally becomes higher. The approaching source compresses successive wavefronts in the direction of travel, reducing the time between arriving wave crests and increasing the measured frequency.
A passing ambulance is the familiar example. Its siren sounds higher than its stationary pitch while approaching, then drops as the vehicle passes and recedes. The change is not usually because the siren itself changes its note; it results from relative motion between source and listener.
The effect was described by Christian Doppler in 1842. For sound, the motion of both the source and listener matters, and calculations must account for the medium carrying the sound, usually air. If the source moves away, the wavefronts spread out and the observed frequency decreases, producing a lower pitch.
“Higher” describes pitch, which tracks frequency for ordinary sounds. It does not mean the sound becomes louder, and the effect is not limited to sound: Doppler shifts also help astronomers study moving stars and aid medical ultrasound imaging.