Sound occlusion in games refers to a process that imitates how sounds are perceived in "closed" environments, closed meaning that there are barriers close to the sound source or listener. How any individual game calculates occlusion (or if it has occlusion at all) varies by game, but the basic principle is that noises should sound different depending on their environment.
Sound waves can travel through/around barriers, but the wave intensity (which the observer perceives as volume) will usually be less than if the wave travels directly to them. This will also change depending on the barrier material, since sound will travel through it or bounce off of it differently.
In the video you linked, you have noticed that you can't hear as much when occlusion is turned on. This is true when the observer doesn't have a line of sight of the sound source, but not when there are no objects between the observer and source. What the occlusion process is doing is reducing the amount of noise the observer hears when there are objects between them and the sound source that "block" the noise.
For more information on the sound occlusion in CS:GO, 3kliksphilip has a great analysis video. Try turning your volume up and listen to how the sound intensity changes as he moves the position of the noise source and adds/removes barriers.