When someone "likes" a comment you have made on another person's status, and you "like" the same comment in response.
This is the first recorded use of the Facebook High Five between the accredited inventor Jason Dominguez, and his friend Gary Wynans, and serves as an example of it's use:
Gary Wynans applaud this man!
9 minutes ago · Unlike · 2
Jason Dominguez The way to applaud on facebook is the double like... it can also be used as a virtual high five... let me show you....
8 minutes ago · Like
Gary Wynans i WISH facebook had a high 5 button
6 minutes ago · Like
Jason Dominguez This is serve as proof we invented it... Don't delete this status... we may be coming into a nice fat check pretty soon!
5 minutes ago · Like
Jason Dominguez UP HIGH!
4 minutes ago · Unlike · 2