GCSE's are a set of exams taken at the end of Secondary School at age 16/17. Most Schools have students take the core subjects of Maths, English, and the three Sciences (Chemistry, Biology, and Physics), and then 3-4 extra classes that students choose from at 14/15. However, some have students taking extra core classes - for example, my school made Maths, English, and the Sciences mandatory, as well as one language class (Spanish, French, or German, that you chose when you first began at the school) and one technology class (Woodshop, Food Tech, Textiles, or Electrical). You were then allowed to choose three extra classes on top of this - I chose Art&Design, IT, and Physical Education (although I now wish I'd taken IT, Drama, and Religious Studies).
In order to get into college - something I think is similar to the last two years of high school - you were required to get at least 5 passing grades (A* to C), and (for the college I went to at least) at least a C in Maths, English, and one of any of the Sciences.
So GCSE's are not pointless, although not passing them does screw you a little in terms of education - or did, as our system is changing now. College used to be optioned, mandatory education ending at 16 with your GCSE's, and they acted as the lowest form of educational qualifications you could possess (A-Levels from College coming after, and then degrees and such from University beyond that). Now, college is mandatory as well, with required education ending with A-Levels at 18/19, and GCSE's are being phased out as they aren't required for entry due to college not being optional.