K im not sure why everyone is worried with prejaculation or pre ejactulation?!
Question: what is it and why is it such a concern
Answers: what is it and why is it such a concern
There are two things you might be talking about so I will just address them both.
1.) "Pre-ejaculation" stands for premature ejaculation, which basically means that the man comes too soon. Since sex is much more enjoyable when it's longer, a lot of people are concerned about this and try to fix it.
2.) "Prejaculate," or "precum," is an entirely different matter. It's a noun, referring to a small amount of semen that leaves the penis before climax. The problem with prejaculate is that men aren't aware that it's happening, though the semen is capable of impregnating a woman. This is one main reason why simply pulling out is not a reliable form of birth control.