could u tell me what 英语四级 is?

could u tell me what 英语四级 is?
hey,i just took a view of the E group .i am so surprised that sb always focuses on 英语四级 subject.so it takes a question out from my mind.is 英语四级 a kind of books or E level?why does my teacher never tell me about it?r there some levels just like 英语四级 in college?who can definitely tell me what 英语四级 is?
thanks a lot!