A prohormone is a precursor to a hormone (like, for instance, testosterone.) Incorporating prohormones into your supplementation regime will boost production of the relevant hormones. Basically, they will prompt your body to produce more of whichever hormone you need it to.
Obviously, this can be a great boon in the fitness world. Often, prohormones are bracketed alongside the likes of anabolic steroids, human growth hormone and testosterone boosters. They can lead to some serious gains, both in terms of hypertrophy (muscle growth) and athletic performance.