What does "On-link" mean on the result of "route print" command?
Yep, the other three answers are correct: it's just a route that's directly reachable (the NIC is in direct contact with it; on the same subnet). To explain a little further though: by contrast, the routes that have a gateway IP listed must be contacted through that gateway.
https://superuser.com/questions/59996/what-does-on-link-mean-on-the-result-of-route-print-command