proof of english proficiency
Frankly, I would suggest you just ask them. There is usually a contact at universities and similar whose role is student services or enrolment support or similar. They should be able to advise you on exactly what is required. Often there is even a dedicated officer for international students.

Taking an educated guess, it may be they require something like a letter of reference, written by a teacher of yours who can attest to your English proficiency, or perhaps academic results from some sort of English class, or an example of your skills, such as a document you have written.

Keep in mind that this is a general question written with the expectation that many applicants will not speak English as their first language. You don't need to prove you're the next Ernest Hemingway, just that you understand English to a sufficient degree that you will not struggle to understand your teachers.