What are standing waves?
Standing waves are the buildup of sound level (increased amplitude) at a particular frequency that results when traveling sound waves coming from opposite directions combine and increase in amplitude (volume). Standing waves occur when sound is reflected back toward the source at the original frequency. This is why it is important that soundproofing materials absorb the sound (by converting that energy into heat) and not simply try to block it from coming through a wall.
Real life example
The amplified sound you hear in sea shells occurs because standing waves are formed as the interior of the shell reflects back the noise of the blood rushing through your ear.
