Diffusion across a cell membrane is a type of passive transport, or transport across the cell membrane that does not require energy. Remember that the cell membrane is a phospholipid bilayer. Although the inside and the outside of a cell are both water-based, there is a hydrophobic region in the middle, and this is an important barrier to anything large, charged, or hydrophilic. Molecules that are hydrophobic, just like the hydrophobic region, can pass through the cell membrane by simple diffusion.
Therefore, simple diffusion is the unassisted passage of small, hydrophobic, nonpolar molecules from a higher concentration to a lower concentration. Very small molecules can slip through the cell membrane, too, even if they are hydrophilic – just like a few ants might crawl through a crack in the wall just because they’re tiny.