Simple diffusion and facilitated diffusion are both forms of passive transport: substances move from an area of higher concentration to an area of lower concentration without the cell spending ATP. In simple diffusion, small nonpolar molecules such as oxygen and carbon dioxide slip through the plasma membrane’s lipid core on their own.
In facilitated diffusion, polar molecules and ions that the membrane would otherwise block move through specific channel or carrier proteins while still following their concentration gradient. Both processes therefore move material down a gradient; the difference is whether membrane proteins are required to help the substance cross.