You see the bar under your name and rank, that's the reputation bar.
The more reputation you get the longer this bar becomes until it's full.
I give rep when I like something, funny post, clever post, great screenshots or videos, etc.
Many times we also use rep to attract a persons attention to a reply post you just made.
I sometimes also give rep to let a person know I appreciate his post, for example when they thank me.
So in that regard it could mean a lot of things.