We wanted a way for members to show appreciating to other members, but we also hate the term "like". This is great for Facebook and those worlds, but we wanted our sign of respect to be similar to the real world.
When someone in the real world does something great, friends give a high five. I mean not everyone does. Some give fist bumps. Some people just smile, but when it's time for a high five it must be pretty epic.
Anyway, a high five is a way to say that you think someones project of contest entry is pretty special.
To add a high five, simply visit a project or contest entry and hit High Five which is located top-right of each project page.