The prostate is an organ that is part of the male reproductive system. It's located under the urinary bladder, surrounding the urethra. It takes in sperm and produces semen, which it then puts in the urethra during orgasm.

Normally it's the size of a walnut. But as the person ages, it tends to enlarge. Surprisingly enough, this is called an enlarged prostate (technically Benign Prostate Hyperplasia). The prostate produces a chemical called prostate specific antigen, or PSA. As it enlarges, it makes more PSA. This can be detected in blood tests, and the amount gives the doctor an idea of how much it has grown.

I mentioned that it surrounds the urethra. As it grows, it squeezes the urethra, making it harder for urine to go through it. This leads to a weaker urine stream, and more urine gets retained in the bladder since it's harder to get it to go through the strangled urethra.