The final ten races of the season are run in a play-off manner. The top 12 drivers in the championship at this stage now compete for the title, everyone else is playing for 13th place in the standings.
As I understand it towards the end of the season the top 10(?) drivers go into the 'chase' which basically means only one of them can win the championship and everyone outside the chase can't. All previous races are then ignored and their points are reset, which means you could win every single race in the run up to the chase but then still finish 10th in the championship. I guess it's a way they choose to spice up the show but it's not something I'd be keen to see in any racing I watch. I'm certainly not sure how interested I'd bee in watching one of the races in the near run up to the chase, a lot of drivers will be just driving to make enough points to win the chase rather than win the race.
It reminds me of the football league playoffs which I also don't like.