As mentioned above by @Bernadette12365, GCSEs are a secondary school qualification, and are below the A-Level qualifications. There are three core subjects: English, Maths and Sciences. Depending on the college you attend they can offer an array of other additional subjects such as: Spanish, French, History, Economics, Geography, Psychology and many more. Students can take as few as seven subjects and as many at 12.
Although your A-Level subjects are what typically determine which University courses you choose to enrol, it's recommended that students' take GCSE subjects that align with their future career aspirations.