Sound waves follow the law of reflection just like light. The angle at which a ray of light hits a mirror is equal to the angle at which it reflects off of the mirror.
In your room, sound waves are bouncing off parallel walls according to the law of reflection. Many of these waves end up reflecting directly over themselves, creating pockets of greatly amplified volume at one frequency, and greatly reduced volume at another frequency, all over the room.
This can mess with your ability to record since your microphones are going to pick up on these waves. But it also interferes with your ability to mix. When you are monitoring, you’re not just hearing what’s coming from your speakers – you also react to what your room adds in and takes away.
A simple example of a diffusor is a curved, reflective surface, like a bent plank of plywood mounted to the wall. When a wave hits the surface, it will still follow the law of reflection — but the angle won’t be the same for any two points along the curve of the surface. So reflections exit at a widened angle about the room, rather than all reflecting off the same flat surface.
This simple type of diffusor will help with standing waves, but it won’t help with comb filtering. Comb filtering is a pattern of peaks and dips in frequency volume that makes your room sound boxy. Better, more expensive diffusers have uneven, reflective surfaces. These surfaces scatter waves unevenly around the room, which results in a much clearer room response.
Diffusors are best used sparingly, to disrupt parallel walls and scatter sound waves without unduly deadening a room intended for both recording and mixing. However, the real key to stopping standing waves and evening out a room’s frequency response are absorbers.