Slim Jim's were seen often opening car doors in 80's movies.
They still are used today, but are probably harder to use with all of the new car door locks, or lack thereof.
A Slim Jim is basically a thin piece of spring steel with a hook on one end. The idea is that you could slide this piece
of metal in between the door and window and maneuver it in a way to hook on to the door lock rods.
Once there you would pull on the rods, which would raise the lock on the door.
So if you lose your keys this is one option to get in your car.
Be aware however that if you don't know what you're doing, you can seriously damage your car.
Moving the Slim Jim recklessly can cause you to break your window, destroy wiring, or mess up the door lock so that even a key won't open it.
So ultimately, learn how to do it for yourself or hire a professional to do it for you.