If you haven't read the books, or played the first two games, don't miss this nice survey article on the PCGamer site: The Witcher 3 story primer: catch up on the essentials.
The Witcher 3: Wild Hunt is a daunting game: a huge, open world RPG and the end of a trilogy starring monster hunter Geralt of Rivia. What do you need to be prepared? First comes brushing up on the events of The Witcher and The Witcher 2, Geralt’s previous video game adventures. Both are big games, dense with characters and political machinations. But that’s not the beginning and end of the Witcher-verse. Geralt’s origins lie in a series of novels by author Andrzej Sapkowski, and guess what? Some important characters from those novels will be showing up in The Witcher 3.
Oh, and uhm: watch your step.