My brother told me: He has sex with men. I (male, 15 years old) don’t understand this. Is it fun?

Yes, it can be fun.

Sex between a man and a woman can be pleasant.
Sex among men can be fun.
Sex among women can also be fun.
There are different kinds of sex.

All people have “erogenous zones”.
“Erogenous zones” are places on the body.
Touching those places during sex makes people feel good.

One of those places is the “prostate”.
The prostate is part of the body.
You can’t see it from the outside.
The prostate is touched by movement inside the bottom.
For example when 2 men have anal sex with each other.
Anal sex in this case means:
One man sticks his penis into the bottom of the other man.
Anal sex can be very pleasant for both men.

Other “erogenous zones” are, for example:
The whole penis, testicles, nipples, or the neck.
When many “erogenous zones” are touched in a pleasing way, sex is fun.