On the extremity of a sperm head (acrosome) can be found an enzyme called proteolyctic which helps the sperm cell to get through the corona radiate layer of the oocyte and create its way to the egg cytoplasm. This process is known as the acrosome reaction.
Once the sperm cell reaches the pellucida layer, the sperm will bind to special receptors letting the sperm passing through.
The sperm cell membrane will fuse with the oocyte membrane and special lysosomes. The cortical granule will then release enzymes which will thicken the pellucina zone so that no other sperm cells would penetrate in the oocyte region. This process is called the cortical reaction. The ovum would therefore only be fertilized by one sperm cell.