When a sperm from a male and an egg from a female come together, they make a zygote - the first cell of a new living thing. This is called fertilization and it's how most animals and some plants reproduce.
So, when a sperm and egg get together, they make a baby! This usually happens inside a lady's body, in her fallopian tubes. After that, the baby starts growing and eventually becomes a fetus.
Lots of stuff can affect fertilization, like when it happens, what the environment's like, and how healthy the sperm and egg are. It's a pretty complicated process that involves a ton of chemical and biological things happening between the sperm and egg. But it's super important because it keeps different kinds of creatures alive and makes sure there's lots of variety on our planet.