Semen extender is a liquid diluent which is added to semen to preserve its fertilizing ability. It acts as a buffer to protect the sperm cells from their own toxic byproducts, and it protects the sperm cells from cold shock and osmotic shock during the chilling and shipping process. The extender allows the semen to be shipped to the female, rather than requiring the male and female to be near to each other. Special freezing extender use also allows cryogenic preservation of sperm, which may be transported for use, or used on-site at a later date.
Read article on Wikipedia