To hijack a vehicle is to take control of it by force. Chances are you've seen an action movie in which some villains hijack a car or plane.
To hijack a vehicle is to take it over illegally. Such a crime is called a hijack or a hijacking. Hijack can be used more generally to mean “take over.” If your friend has a bad habit of interrupting other people to talk about himself, you can say that he tends to hijack the conversation.