An abbreviated version of the meme "2/10 Would Not Bang" which is giving a picture of an extremely attractive woman a 2 out of 10 for minor flaws. It is meant to be a joke and even a compliment that the person is so attractive, giving her a 2/10 is obviously not a serious rating.
The second letter of the alphabet, the 10th letter of the alphabet... if you are unfamiliar with that, here's the alphabet, with the second and tenth letters capitalized...
a B c d e f g h i J k l m o p q r s t u v w x y z.