Functions refer to what items of language actually do in a real context, as opposed to what they might mean literally. These include suggesting, criticising, refusing, agreeing and disagreeing, enquiring, talking about the past, and giving advice.
The phrase 'What time do you call this?' has a clear literal meaning but its function is to ask 'Why are you late? I'm very angry!'
In the classroom
It is important for learners to understand that one form may have many different functions, and to see how functions work in context, as the example above shows. Many forms have sensitive functions and so their appropriacy, formality and degree need to be considered.