Sometimes, we use "summit" as a verb, to mean "to climb all the way to the top," like this: "You must be strong and experienced to summit Mount Everest."
Most of the time, "summit" means "mountaintop." However, we can also talk about the summit of a hill, the summit of a tower, the summit of a heap or a pile of things, or the summit of a house or a castle. Check out the weather vane at the summit of this house.
We can even talk about the summit of a period of time, meaning the "highest" part of it, in the sense that it's the best, the most exciting, or the most accomplished. For example, if you're at the summit of your career, you're at the "highest" part: the part where you're the most successful, doing your best work, and earning the most money. Or, if you're at the summit of your fame, you're at the "highest" part: the part where you're the most famous.
Here's another way we use the noun "summit." When the highest, most powerful leaders of two or more countries get together to have a meeting to talk about important things, we call that meeting a summit. It doesn't take place on a real mountaintop! It just involves the "highest" or most powerful leaders of the world. Here's a photo from a 2021 summit, attended by many of the world's presidents and prime ministers.