Acoustic resonances within an enclosed space or room. These occur at specific frequencies where the source sound is reflected from the room's boundaries to reinforce and/or cancel with itself to create standing waves. This results in some areas in the room with very boomy or exaggerated pitches, and others where the pitch may be almost completely absent. The resonant frequencies involved relate directly to the sound wavelength and room dimensions, and is particularly prevalent at low frequencies.
Here's a handy video from GIK Acoustics, explaining 'room modes' and offering some treatment solutions.