Diffusion is the random motion of molecules from an area of higher concentration to an area of lower concentration.
Molecules tend to move constantly in solution and spread out randomly. When the concentration of particles in solution is spread evenly throughout, the solution is at an equilibrium.
In a cell, the membrane creates a selectively permeable barrier between the interior of the cell and the exterior surroundings. If there is a difference in concentration of a solute on one side of the membrane, diffusion will occur from high to low concentration until equilibrium is achieved. This explains how oxygen (in higher concentration outside the cell) enters the cell by diffusion through its membrane.
It is important to note that the cell doesn't expend any additional energy for diffusion to occur. It is an example of passive transport.
I have included a link which has an excellent diagram showing diffusion.
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