The fluid is called pre-ejaculatory fluid or pre-cum, and it comes from the Cowper's gland inside a guy's reproductive system. It's released to clean out the inside of the urethra when a guy gets an erection. The urethra can have leftover urine in it from an earlier trip to the bathroom, and urine is acidic enough to be unfriendly to sperm.

The fluid makes the urethra less acidic and gives the sperm a better shot at surviving the trip from the guy's testicles out of the body. Pre-ejaculatory fluid does not contain sperm. But if a guy ejaculated recently and live sperm are still inside his urethra, the fluid can “pick up” the leftover sperm on its way through the urethra and out of the penis.

For example, let’s say that a guy has sex and ejaculates at 9 p.m. He doesn't go to the bathroom after he has sex. At 10 p.m., he gets another erection and pre-ejaculatory fluid is released. That fluid can have live sperm in it. If a guy pees after he ejaculates, that would kill the leftover sperm.

Also, remember that pre-ejaculatory fluid can carry sexually transmitted diseases (STDs), so a guy should put on a condom as soon as he gets an erection.