What is diffusion and how does it occur?

is the net movement of molecules from the region of higher concentration to region of lower concentration.
Diffusion is a passive process. It is like when you open a bottle of perfume, soon the whole room will be covered with its smell. From region where the concentration is high, the molecules move to the regions where the concentration is low down the concentration gradient unless equilibrium is reached.
Here is a video which illustrates how diffusion can occur across a semipermeable membrane.