GCSE (General Certificate of Secondary Education) is the basic academic qualification that kids take around the time they're 16, usually after a two-year course, although adults can take them after a one year course in community/further ed.
Instead of it being one all-encompassing certificate, you take one GCSE per subject. Some subjects carry the equivalent of 2 GSCEs - science is typical, and if kids tell you they're doing 'double science', then that's what they mean. Most kids do a maximum of around eight or nine subjects. The benchmark pass rate is to have grade A* (called 'A star') to C in at least five subjects, although the grades go down to E or F, I think. The idea is that there's no 'pass' or 'fail' with them - you just get a grade if you do the work. Once they've got decent grades, then kids can go to do A (advanced) level qualifications in three or four subjects, which may or may not be subjects they've taken at GCSE. It's the A levels that are used by university admissions people to determine entry.
Although the education authorities would have us believe otherwise, GCSE results are no measure of how well a sixteen-year-old will perform in a job.