It tells you what your reputation is, which is determined by how other members of the forum think of you. Right now you at 10 rep because no-one has added or taken away any rep yet, thus it is green.
And to answer your next question which I believe will be 'How do I get more' you get more rep by other people repping your post because they liked it or thought it was amusing.
There are about three threads on this, I'd advise using the search option first next time
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