The prostate gland manufactures an important liquefying component of semen.  Sperm are produced in the testicles and then stored just behind the prostate in the seminal vesicles.  Here, and at the time of ejaculation, the sperm are in a jelly-like medium. At orgasm and ejaculation, the prostate and seminal vesicles contract, mixing their respective contents.  The fluid in the prostate contains large amounts of a substance known as prostate-specific antigen (PSA), which liquefies the previously gelatinous sperm mixture, allowing the sperm to move freely in search of an ovum to fertilize.