A parking lot is like a big space where cars can "rest" when not being used. Think of it like a big playground for cars. Just like how when you sleep at night, you need a bed to rest on, cars need a safe and comfortable place to rest too when not being driven.
People go to parking lots when they need to leave their cars behind for some time, like when they're going shopping or to work. The parking lot normally has a big sign outside with the name of the lot, and there are many places to park your car inside.
When you arrive at the parking lot, you will see different lanes with arrows and numbers painted on the floor. These help show you around and to help you choose where to park your car. Sometimes there are other cars in the spots near the entrance, so you will need to look for empty spots further away.
When you finally find an empty spot, you would park your car in it carefully so that it's straight and so that nobody else is blocked. Then you can go do whatever you need to do, like go to work or go shopping.
Finally, when you come back after you're done with your tasks, you can get back in your car and drive it out of the lot. It's important to be careful when you're driving in a parking lot because there are lots of other cars and people walking around.
That's pretty much what a parking lot is. It's a safe and convenient place to leave your car when you don't need it, so you can go do your other activities.