First a correction: Your question should be, 'What does whether mean in this sentence?" I agree with the other explanations. To make it simple, the phrase in your question could also be worded, "if their English is at required level."

Do you have any idea whether their English is at the required level ? > Do you know (anything about ) if their English is at least of good enough standard necessary.

'whether' introduces two possibilities. Often one of the possibilities is not written or spoken, but implied. The two possibilities here are 1) their English is at the required level, or 2) their English is NOT at the required level. Often, 'whether' can also be written as 'whether or not'. Example: 'an idea of whether or not their English is at the required level'.