A lot of what we teach is so the students can produce English for a specific purpose, for example to order food at a restaurant. This is how we describe Functional English. Functional English plays a major part of TESOL.
Remember, the difference between TEFL and TESOL is that TESOL is more for immigrants / refugees living in a country where English is the native language and TEFL is for teaching English for students living in non-native English-speaking countries where they are learning English for work, to be able to study, or as a hobby. You can imagine that, because TESOL is for people living in a native English-speaking country, it puts a lot of emphasis on functional English because TESOL is a tool to help them integrate into society (open a bank account, go shopping, pay bills, complain etc.). Having said that, there is a lot of overlap, some skills are good for everybody to learn. Ordering food, complaining, asking for directions, are all good skills to have for any tourist on holiday.
Some of the basic functional skills we want to teach are:
Asking for information
Offering to help
Intentionally being vague / non specific
Saying no politely
Making a complaint
Guessing / predicting
All functions are taught as though teaching vocabulary. You introduce and explain the meaning, you make sure the students understand and can pronounce the phrases, you practise using them with gap fills etc. and then you give the students the chance to use them in a more realistic environment, for example role plays.
Here are some of the basic functional skills in a little more detail: