To hitch hike is to travel by standing on the side of the road and soliciting rides/lifts from passing vehicles. In order words, you get a lift by openly asking for it.
A related term, because of the sign up often used when hitch-hiking, is 'to thumb a lift'. Thumbing a lift can be used of a more incidental form of 'soliciting a lift': His train was cancelled, so he thumbed a lift, but During the year before his course started, he hitch-hiked around Europe..