Intensifiers are words that make the meaning of other words stronger.

Example
‘She's very good at maths' and ‘You are much taller than your brother'.

In the classroom
One high level function of intensifiers is in sarcasm, for example:
‘Oh, yeah, that was really clever'
Learners at higher levels can look at this use of intensifiers, especially if they are in an English-speaking country. It is important that they can recognise this function.