Assuming you just moved to the uk from an international country, you should already be given guidance on gcse, but here is a summary:
GCSE stands for General Certificate of Secondary Education and these exams are done during May of Yr 11.
In GCSE, there are two types of subjects you will do: core subjects (these subjects are mandatory) and optional subjects (these subjects are the ones that you picked before starting gcse).
Core subjects in the uk generally include Maths, English Language, English Literature and Science.
Optional subjects in the uk are around 4 subjects and it depends on what your school offers - you can do geography, history, languages, business studies, sociology, art, drama, music, computer science and so many more.
In your school you may be offered to do extra gcses, like gcse further maths or gcse triple science - during year 10, you do something called combined science and if you do well or are interested in science, you are offered triple science to do in year 11 and this is more difficult, but you leave with 3 gcse qualifications instead of 2 (which you would get if you pick to do double science).
GCSE grades use the 9-1 system. 9 is the highest gcse grade you can get and U means failed - most of the time, getting a gcse grade of 4 or 5 is a pass.
Every gcse subject has an exam board (common ones include AQA, Edexcel, Eduqas, OCR and WJEC), these exam boards have their own curriculum and different ways of running your exams. For example, AQA maths and Edexcel maths both have a different curriculum covering different topics and have different ways of examination. You need to check your school's curriculum to see which exam boards you have for your subjects.
If you hear something called coursework, it just means that you will be examined throughout Year 11 through mini examinations or assignments and the grades you get in these add up to make your final GCSE grade. If your subjects don't have coursework, it means that you will only be examined through your final exam in May and the grade you get from the final exam will make your final GCSE grade.
If anything I said doesn't make sense, you can do your own research on what gcses are, but I just gave a summary.
I hope that helps and good luck for year 11!