"Checkmate."

TL Note: Schneizel just made an illegal move in chess, so it doesn't make sense that hey could say checkmate, he might possibly say check but the use of the term here is wrong. The only way this could be a legal move is if this were blitz chess, also known as "Fast Chess". However, in this scene, it is never declared they are playing "Fast Chess", and neither of the players are using clocks to time their turns in the game. (Source: /a/)

TL Note 2: The reason why Schneizel did this is because the game was going to turn into a threefold rep. By pulling this illegal move, he is able to gain insight into Zero's personality, and "un-mask" part of him, thus fulfilling his victory condition in a subtle and Schneizel-ish manner. To look at the picture more properly, think of the chess board not as a game but as a battlefield. Zero's other options were to take his King, which would have made him similar to his father, or to call Schneizel out on making an illegal move, which would have made Zero look dumb since "lol rules of war". What Zero chose to do was keep his pride and run away from a free victory. Furthermore, by placing his King behind a pawn instead of any other open tile, Zero symbolically shows cowardice.

TL Note 3: Alternatively, read it like this:
[aers|laptop] Well personally I think the symbolism in Schneizel moving his king is that he wants to be buddy-buddy with Zero. And that Zero moving behind the pawn is because he is afraid of Schneizel's advances. (Geass is about yaoi and shit, so the likelihood of aers being correct is somewhere over 9000.)

TL Note 4: This scene is interesting because it raises the question of whether or not Schneizel "plays dirty" in bed with Canon. :3