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. Room modes are one of the biggest obstacles to accurate sound reproduction. One way to describe standing waves is when sound bounces back and forth between two parallel walls. Sound gets trapped between the two walls either attenuating or boosting the frequency. When a frequency gets trapped then so do its harmonics. An Example of 50 Hz would have 100 Hz,150 Hz ,200 Hz ,250 Hz, and so on trapped. By adding absorption you can dampen the standing waves, which will result in better mixes. Sometimes dispersion cylinders are carefully placed around the room to break up modes. Some people try to use equalizers connected to their speakers to compensate for these standing waves. It is said that since most equalizers have there own quality penalty compared to absorption and that the room responses varies in relation to different positions with in the room that they are best not to be used. You have to remember that you still have sound reflecting between the walls. Diffusers are often used in small rooms to target low frequencies. Bass traps are used to smooth out low frequencies and help in reducing the decay time, which tend to be needed in smaller rooms. Flutter and echo are caused by mid and high frequencies bouncing between two parallel surfaces. Absorption is the most practical for this as well. It is important you know these things exist and that there are solutions for your room that will help you create better mixes.
Izhaki, Roey; Mixing Audio; Concepts, Practices and Tools; Second Edition