Car umbrellas are portable tent-like structures that fit on the roof of your car. They open up and protect the paintwork from dust, debris, rain and snow. They also provide protection from the damaging effects of UV light. They create a layer of air just above the car roof and so this insulates the car from heat. On a hot day, it can reduce the temperature in the car by tens of degrees and makes it much more comfortable to get in it.
There are other types of car umbrellas. These fit onto a car or truck hitch and look much like a very large regular umbrella. They are used to provide shade for outdoor activities such as picnics and work projects.