Ok kiddo, so imagine you have a toy car and you leave it in your room. Your mom can't just go in your room and take it without asking, right? Well, for adults, it's kind of the same thing but with real cars.
The motor vehicle exception is a rule that says that police can search a car without a warrant if they have a good reason to believe there is something illegal inside the car. This is because cars can move very quickly and can be driven far away, so police need to be able to search them fast to keep themselves safe and to prevent any evidence from being destroyed.
For example, if a police officer pulls over a car for speeding and sees drugs or a weapon in plain view, they can search the car without a warrant. Or, if someone is driving recklessly and they suspect there might be evidence of drunk driving in the car, they can search it without getting permission first.
However, just like with your toy car, police can't search a car for no reason. They need to have a good reason and it's up to a judge to decide if their reason was good enough or not. So, the motor vehicle exception is there to help police catch bad guys but also to protect our rights as citizens.