To hitchhike is to get a free ride from a passing car. If you see someone standing alongside the highway raising a thumb, chances are that person is hitchhiking, not just expressing approval of your driving style.
In the past, it wasn't unheard of for people to hitchhike across the country, but it's much less common today. If you hitchhike, you typically stand beside a road or highway with one thumb pointed up. The hope for a hitchhiker is that a car will stop and its driver will provide a free ride. While for many people, hitchhiking was once considered a reasonable way to travel, today it's illegal in some places and generally thought of as dangerous. The word hitchhike dates from the 1920s.