GCSEs are qualifications that you need to pursue a career or do further study at a college and, if you want to, complete a degree at a university.
GCSEs involve attending lessons, completing additional tasks at home (homework) and (in very few cases) completing coursework.
Your final grade will be mostly, if not entirely, dictated by how well you do in exams that you sit in the last year of your GCSEs.
Nearly all jobs and career paths require at least a 4 grade (equivalent to a C in the old system) in both English and Maths.
To complete a level 3 qualification, you usually need to pass at least 4 GCSEs with either a 4 or 5 grade.