The bell tolling twelve times signifies the end of President Snow's speech, and the beginning of the horrifying reality of the Quarter Quell for the victors. It means that Katniss Everdeen, Peeta Mellark, and the other victors are now forced to return to the arena to fight for their lives once again.
The twelve tolls create a tense atmosphere, making the audience understand the severity of the situation for Katniss and the other victors. It signals the beginning of a new, and even more dangerous, chapter in the story.