The Chamber Of Secrets was a Chamber in Hogwarts created by the wizard Salazar Slytherin and guarded by his pet The Basilisk. The Chamber was not opened until The Heir Of Slytherin discovered it in a girls bathroom and a girl was killed. It is built underneath the dungeons of Hogwarts.
The location is mentioned numerous times throughout the film, until the actual location is seen in the last third of the film. After Ginny Weasley disappeared into the chamber of secrets, Harry, Ron and the cowardly Gilderoy Lockhart went down there to rescue. However, Lockhart attempts to erase Harry and Ron's minds, but fails due to Ron's wand that he is using backfires, causing a cave-in, separating Harry from Ron and the mind-wiped Lockhart.
Harry travels into the actual chamber of secrets and finds the unconscious Ginny Weasley. The 11-year old Tom Riddle appears and reveals all the things that he manipulated Ginny and had her under his spell, and also reveals that he is Lord Voldemort. After summoning the Basilisk, Fawkes arrives and blinds the serpent, allowing Harry to escape from it and luring it away into the sewers.
After Tom Riddle reveals that Ginny will die slowly, the Basilisk returns shortly. Harry draws out Godric Gryfindor's sword from the Sorting Hat, and slays the beast with it, fatally poisoning and wounding himself in process. Harry uses its fang to stab Riddle's diary, disintegrating the 11-year old Riddle from existence. After Ginny awakens and Harry is healed back to his strength from Fawkes' tears, they escape the chamber with Ron and Lockhart.