It's an ECO (Encyclopedia of Chess Openings) code showing which opening line you are following.
More information: http://en.wikipedia.org/wiki/Encyclopaedia_of_Chess_Openings
List of openings with ECO codes: http://en.wikipedia.org/wiki/List_of_chess_openings
I am curious about the little designation that is in front of or after openings when I'm looking at games. Example: Sicilian Defence B92 or Queen's Indian Defence E15. What does the B92 and E15 refer to? Thank you!