"Getting pulled over" specifically means that a police car forces you to stop your car because you've been doing something wrong. When you "get pulled over", the police come behind you and turn on their lights and siren. When that happens, you're supposed to pull over and wait for the police officer to come to talk to you. Depending on what you were doing, the police officer might do one of the following:

• give you a warning
• give you a ticket
• arrest you