Phospholipds are molecules with a polar(hydrophilic) head and and a hydrophobic tail. A phospholipid bilayer is the special type of membrane in cells with two layers of phospholipids with tails bonded to each other and polar heads are facing away.
This arrangement is very much critical for the cell structure.
THe primary role of the bilayer is to separate the aqueous material from the surrounding environment. The surrounding environment of the cells are also aqueous and filled up with hydrophillic molecules and if there is no partitioning any molecule can go into the cells. This prevented by the hydrophobic bilayer in the middle of this arrangement. The hydrophillic molecules have a very little or no chance getting through this hydrophobic bilayer in the middle.
Also this phopholipid bilayer is filled with proteins which allows it to maintain its fluidity and adjust to the different conditions in the environment.