Controlled language is language that the teacher gives the class that is in some way changed to suit the level or the aims of the lesson. This contrasts with authentic language, which is not adjusted in any way.
A teacher may prepare a written biography of a famous person that only uses present simple tense, for an elementary class.
In the classroom
There is debate over the use of controlled and authentic language. Many teachers favour an 'in at the deep end' approach, where the target language is within a text that is not changed.