Block is to prevent polyspermy.
FAST BLOCK:
In marine invertebrates, including the sea urchin, a fast block to polyspermy occurs within a tenth of a second of fusion OF EGG WITH ONE SPERM. The fast block to polyspermy involves the opening of Na+ channels in the egg plasma membrane. Na+ flows into the egg cell,depolarizingthe membrane. This depolarization prevents additional sperm from fusing to the egg plasma membrane.
SLOW BLOCK:
The slow block to polyspermy begins within 10 seconds of fusion of the sperm and egg plasma membranes. A compound called inositol triphosphate (IP3) causes the release ofCa++from intracellular stores in the egg endoplasmic reticulum. Ca++ is first released at the site of sperm entry. 
This Ca++ results in the fusion ofcortical vesicles ( surrounfd the egg) with the egg plasma membrane, releasing their contents into the space surrounding the egg, called the perivitelline space. This raises the vitelline membrane, and inactivates receptors on the vitelline membrane.