When a guy cums fast, it means that he has reached orgasm relatively quickly compared to other guys. There are a number of factors that can contribute to a guy reaching orgasm quickly, including physical factors like how aroused he is, how much stimulation he's receiving, and his overall level of sexual experience. Additionally, some guys simply have a higher rate of orgasm than others. While there's nothing wrong with reaching orgasm quickly, some guys may feel like they need more time to fully enjoy sex. If this is the case, then it's important to communicate with your partner and make sure they're okay with things moving at a faster pace.