Diffusion is how things like atoms or heat can move through materials.
You can see it in action. Say you pour hot water on some sugar in a clear mug. At the bottom the sugar dissolves, but there’s still more sugar at the bottom of the mug: you see some wavy lines where the light you see is going through the sugary water differently from the non-sugary water. If you sit and watch it, you see that the wavy line moves up from the bottom to the top and becomes less and less clear.
What’s happening is that the molecules of sugar are making bouncing around randomly with the water molecules that are around them. Some make move toward the surface of the water, and some might move away. After some time, you get more and more sugar molecules towards the top.
Diffusion is really just the 2nd law of thermodynamics: the disorder ( the entropy) of any isolated system always increases.
So if we have a box with two compartments separated by a partition, one side is filled with gas and the other a vacuum we have a system. In the half with gas there is disorder, gas molecules floating about all over the place bashing into each other and tumbling around. In the other half there is essentially no disorder. There is nothing, a vacuum. If we take out the partition, the gas will diffuse (or move into by simple RANDOM motion) into the vacuum and we have a new system. In our new system, there is overall more disorder: There are now gas molecules floating about all over the place bashing into each other and tumbling around over THE WHOLE BOX not just one half of it. We can think of diffusion as a process where things go from a high concentration to a low concentration until it is balanced but really its all just entropy.