Dissolved or gaseous substances have to pass through the cell membrane to get into or out of a cell. Diffusion is one of the processes that allows this to happen.
Diffusion occurs when particles spread. They move from a region where they are in high concentration to a region where they are in low concentration. Diffusion happens when the particles are free to move. This is true in gases and for particles dissolved in solutions - but diffusion does not occur in solids.
Particles diffuse down a concentration gradient, from an area of high concentration to an area of low concentration. This is how the smell of cooking travels around the house from the kitchen, for example.
Products of digestion, dissolved in water, can pass across the wall of the small intestine by diffusion. Their concentration is higher in the small intestine than their concentration in the blood, so there is a concentration gradient from the intestine to the blood.
Oxygen and carbon dioxide, dissolved in water, are exchanged by diffusion in the lungs:
The dissolved substances will only continue to diffuse while there is a concentration gradient.