Sexual activity is when two people or more touch each other's bodies in certain ways that make them feel good. This can include kissing, cuddling, or touching each other's private parts.
Boys have something called a penis and girls have something called a vagina. When these parts touch or go inside each other, it can feel very pleasurable. This is called sex.
But sex can also make a baby, so it's important for grown-ups to be careful and use special things called condoms or birth control to make sure they don't make a baby before they are ready.
It's also important to remember that sex should only happen between people who want to do it and they should always talk about it and make sure they both feel comfortable and happy with what they are doing.