"Men who have sex with men" means that sometimes men like to have romantic and/or physical relationships with other men. This is different than what most people see as a traditional relationship between a man and a woman.
When two men have sex, they might do things like kiss, touch each other's bodies, give each other oral sex, or have anal sex. These are all ways that two people can show love and affection towards each other.
It's important to remember that having sex with someone else is a personal choice, and as long as everyone involved is a consenting adult, it's nobody else's business. In many places, it's also protected by the law, meaning that people can't be discriminated against or punished for their sexual preferences.
If you or someone you know is interested in exploring their sexuality and wants to have sex with someone of the same gender, it's important to always use protection (like condoms) to prevent the spread of sexually transmitted infections (STIs) and to communicate openly and honestly with your partner about your wants and needs.