To trespass is to illegally enter someone's property or overstep your bounds in another way.
Have you ever seen a "No trespassing" sign? If so, you probably know it means "Keep out" and that trespassing is to go somewhere unlawfully. If you break into a store that's closed or enter a stranger's yard, you're trespassing and could be arrested. Also, trespassing refer to other types of stepping over boundaries. If you lie to a friend, that's a type of trespassing. Taking advantage of someone is trespassing. Both kinds of trespassing are violations.