Phospholipids are the primary molecules found in the plasma membrane. A phospholipid is unique in that it has a hydrophilic region called the head, and a hydrophobic region known as the tail. The hydrophilic region can interact with water due to the presence of a phosphate group which is polar, like water. If something is hydrophilic, that means it likes water. Conversely, the hydrophobic region of the molecule does not interact with water because it is not polar. If something is hydrophobic, that means it does not like water. A molecule that has both partial hydrophilicity and partial hydrophobicity is classified as amphipathic.
The Need for a Lipid Bilayer
The interior of the cell is primarily made of water. Likewise, the exterior of the cell is usually surrounded by watery fluid. This means that the plasma membrane could not possibly consist of just one layer of phospholipids. This is because the hydrophobic (or water fearing) tail region would have to interact with one of the watery regions inside or outside of the cell. So instead, the cells have evolved to have two layers of phospholipids.