The sexual double standard refers to the popular notion or belief that men who engage in multiple sexual relationships with different partners before marriage are treated differently that women who display the same behavior—men are usually praised, while women are judged. For example, a young woman who has had several sexual partners in one month is often regarded as sexually immoral, while a young man of the same age who has had several sexual partners in one month is regarded as someone worthy of praise and respect.
Another example of the existence of the sexual double standard is the understanding and conceptualization of virginity. Men who lose their virginity early and continue to engage in serial sexual relationships are praised, while women are often put in a complicated situation in which losing their virginity early and continuing to enjoy their sexual freedom means that they're immoral and "easy," while choosing not to lose their virginity and/or not to continue sleeping with different partners means that they're prude.
Men are allowed greater freedom when it comes to sexual relationships than women mainly due to sexism, such as the archaic sexist belief that men are stronger and more powerful than women and therefore deserving of more authority and freedom, as well as due to the existence of gender norms and roles, in which both men and women are traditionally expected to behave in certain ways.
While the sexual double standard still exists today, especially in more conservative societies, it is important to mention that the situation is a bit different than the past. More progressive societies are not as judgmental toward the expression of sexual freedom of people of any gender as before; even if an individual doesn't approve of such sexual behaviors, they might choose to keep that opinion and mentality to themselves, as "slut-shaming" is often criticized nowadays.
It is a fact, however, that the sexual double standard in which men enjoy more sexual freedom than women unfortunately still persists even today. In this context, it's also notable to mention that young men tend to praise male sexual behavior, while young women tend to praise female sexual behavior. In the past, women often shamed other women who had multiple premarital sexual partners (specifically male sexual partners), but today, women don't have as much prejudice toward other women who freely engage in sexual relationships with others as they did in past times.