The teaching fields require extensive and specialize training. There are many different types of teaching that's why the field is very high. For example TESOL is to advance professional expertise to teach English to other speakers around the world and the speakers will teach the teaxhers how to speak their language.
Different teachers travel all around the world teaching the language of English.
Teaching children English is showing them their second language or a foreign language.
This picture represents that a teacher is teaching how to say Hello in English.