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.