The same thing it means everywhere else - it's a social network _minus_ kids and housewives.
(Also househusbands)
LinkedIn recently tried to get me to connect to a high school girl. This wasn't a case of a minor lying to get access to a service, her profile clearly stated that she's still in high school. Note that this wasn't her trying to connect with me, it was LinkedIn's algorithm suggesting it. My take away is that there are minors on the system and the system isn't doing much to protect them. There was nothing in her work experience that overlaps with my career or interests and we had no connections in common. Seems very odd that LinkedIn would suggest this connection.