It is important for students considering the study of English to understand the term 'metalanguage.' They need to scrutinze the sylllabus carefully to make sure it will suit them, and their particular interests. For example, I knew a few students who were bitterly disappointed with their degree choices as they were passionate about literature, but their courses revolved around analysing language, not reading novels and writing essays about them. Language and 'metalanguage' can be a fascinating topic - but only for purists.
"Meta" is a Greek prefix, used to indicate analysis of how something works. For example, "in linguistics a grammar is considered as being expressed in a metalanguage, or a sort of language for describing another language (and not itself)."
When English teachers are teaching the use of different verb tenses and providing explanations of how and why those different tenses are appropriate to different sentence construction patterns, they are using metalanguage.
Metalanguage is the use of a specific language or vocabulary in regards to describing or analyzing language itself. It is important in teaching English in regards to the way that one would use the native language in order to teach the second language (or the language which is being learned/taught).