In pro wrestling, basically he's the one running the show. He decides who's winning the matches, how the matches will end, and all the various storylines and feuds that lead to the matches taking place.
Bookers can be all kinds of different people, from the owner of the promotion (Paul Heyman in ECW), wrestlers for the promotion (Kevin Nash or Kevin Sullivan in WCW), or people hired by the promotion for the job (
Jim Cornette in Ring Of Honor). Booking can also be done by committee, the current
bookers of the WWE are owner Vince McMahon, "head of creative" Stephaine McMahon-Levesque (Vince's daughter), "head of talent development" Paul "Triple H" Levesque (semi-retired wrestler and Stephanie's husband), and various other people. The term "book" can also be used as a verb, as in "I wish they'd book Daniel Bryan to actually win a
match one of these days" or "WWE TLC 2012 was booked very well".
Jim Cornette was fired as
Ring of Honor booker in October 2012, and replaced by ex-wrestler Delirious.