IB stands for "International Baccalaureate". You spend 3 to 4 of your High School years doing (on average) double the work-load of your non-IB friends.
You then spend your Senior year of High School stressing over your final exams. If you pass them all, you might get credit from colleges you apply to. Also, it apparently makes it easier to get into schools. Knox currently awards one-year of credit to High School seniors who have an IB Diploma score of 33 or higher.