What does being reported for "Openings book" mean?

I was playing an opponent and he messaged me saying "reported for openings book btw" What does this mean? I typically play the Alekhine's Defense, so idk if that offended him or what? I have attached the game 

The guy probably needs to have his head examined, that is the most likely explanation.

Also: this is unrelated to the question, but you should look at move 21 of the game. After white's ridiculous Ne5 blunder, you had an easy win with Qxd2+ 2. Kb1 Nc3+ 3. Ka1 Nxa4. But you did not take it. You should think about why you didn't notice it.

It's only a problem if you have the book open in front of you while actually playing the game. In daily games, though, it's not only permitted but expected. Players may use any resources they like aside from computer assistance.

If this is from an opening book, it needs a rewrite.