Room modes are the collection of resonances that exist in a room when the room is excited by an acoustic source such as a loudspeaker. Most rooms have their fundamental resonances in the 20 Hz to 200 Hz region, each frequency being related to one or more of the room’s dimensions or a divisor thereof.
Source – Wikipedia

A room mode is a resonance that occurs in a closed room.

The resonances will occur when a room dimension is a multiple of a certain frequency’s half wavelength. The result is that high pressure zones and low pressure zones will always be at the same place, lengthening dramatically the resonance time.

There are dozens of 3-dimensional room modes in any room. They will typically resonate much longer than other frequencies and be very detrimental to good listening condition.