For some children, exams are particularly difficult because of their level of reading skill, and for this reason a reader is allowed to help a specific child in a GCSE exam. The children who need this help will have been identified by the school well before you reach GCSE age, and are normally children who are on the Special Educational Needs register. The school has to justify the use of readers to the exam board, and so have to keep records of why they arrived at the decision to use a reader for a specific individual.
A reader is employed by the school to sit on a one to one basis with an individual child and read the exam paper to them. The child will be given extra time to allow for this disruption, and will take the exam in a room with others in the same position. Your reader can only read the words that appear on the paper, and cannot explain meanings of words or questions. Should you ask they will remind you gently that they cannot answer that question. You can ask them to re-read a question as many times as you need.
If you are writing for yourself you can then answer the question as normal, and then ask your reader to read the next question for you, and so on. You can point to anything you need read. For some people their reader is also available as a scribe, and so writes for you as well.