What is a sixth form college?

It is a college in the UK usually for ages 16-18 year olds. It is a continuation of secondary education where students can follow more advanced qualifications such as A levels (otherwise known as General Certificate of Education Advanced Level) or equivalent which would give access to university courses (higher education).
Some secondary schools would offer these courses themselves but in some areas a sixth form college would accept students where the local schools did not offer these.

Sixth form colleges are not only known in the UK but also in parts of the Commonwealth (Brunei, Caribbean).

https://en.wikipedia.org/wiki/Sixth_form_college