UCAS tariff points are points used to determine your eligibility for a course. Each grade you receive in a specific qualification is given a weighted-point average, these are then accumulated at the end of your application, and the resultant total determines whether you have reached the entry requirements. As for working them out, there are lots of ways to do this. UCAS has a very handy UCAS tariff points calculator, which tells you exactly how many points you have.