So if we start off by looking at the structure of a typical phospholipid inside a cell membrane, we can see that it is made up of two components: a hydrophilic phosphate head, which means it is 'water loving' and attracted to water, and hydrophobic fatty acid tails, which are pointing inside the membrane and are not attracted to water. These phospholipids will line up next to each other to make the cell membrane known as a phospholipid bilayer.
Phospholipids can help decide what molecules will pass through the membrane, and into the cell. Molecules that are hydrophobic can pass though the membrane because the tails are also hydrophobic. However, a hydrophilic molecule, would only be able to pass the head by itself, because they are water-loving like the exterior of the membrane, and are therefore excluded once they reach the hydrophobic tails.