When something is victimless, no one is harmed by it. If you accidentally run a stop sign on an empty road and nobody gets hurt, it makes sense to consider it a victimless crime.
The adjective victimless often refers to a punishable crime that didn't really hurt anyone; in other words, there's no victim. For example, it might be illegal to loiter in front of the library at night, but it's really a victimless offense. This word comes from the Latin root victima, "person or animal killed in sacrifice," and -less, "free from."