Although there are a number of locations in Harry Potter and the Chamber of Secrets, the most important setting is the one that is shared by most of the novels: Hogwarts University, the school where young children with magical ability are taught to control and utilize their gifts.
Hogwarts is a massive complex, designed superficially like an English castle but with enormous grounds and an interior that is magically larger on the inside. Hogwarts is typified by its interior, which uses magic to create impossible structures, staircases, and passages. Author J.K. Rowling mentions that Hogwarts could not have been built using conventional construction methods, because the impossible architecture requires magic to exist (Wikipedia). The school has been around for a long time, so many rooms become significant to the plot of the story.
The other most important location is the Chamber of Secrets itself. This is an underground cave that has been sealed off and forgotten for years; only a person who speaks the language of snakes can open it up. The Chamber hides two major secrets: a basilisk, which starts to kill people during the second book, and the diary of Voldemort, which is shown later to house a Horcrux. The characters return to the Chamber years later to retrieve basilisk fangs.