5 GCSEs at grades 9-4 (or A*-C under the old grading system) is generally considered to be the equivalent of a US high school diploma.
GCSEs are 2-year courses taken between age 14 and 16 in the UK, usually in about 9 subjects, including English, Math, Science, a Humanity subject, a language, a technology subject and then a couple of others.
A passing grade is a 4 (a C under the old system, which only changed a year or two ago) and so 5 GSCEs grades 9-4 or A*-C means that you got a passing grade in at least 5 subjects (usually they want 2 of those subjects to be English and Math, sometimes Science as well).
If you took AP classes in high school, college prep classes, or you have an Associates Degree, those are generally considered equivalent to A levels, which are 2-year advanced level courses taken in 3-5 subjects from age 16-18. A-levels (or equivalent) are the academic pre-requisites to get into a UK university
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