The waiter put the lemon into the water burn it with the match and covered the glass over the lemon so when the lemon and the match and burns it takes up oxygen and produces carbon dioxide and thus, the volume of gas in the glass would decrease and the water in the plate would be sucked into the glass.
Or something like that?
Edits: AHHH the lemon is for the match to stick out of the water! I think he puts the lemon into the water and stuck the match into the lemon so that the head of the match does not touch the water?! o.o correct?