Formulaic Expressions are expressions (sentences or phrases) that come almost automatically after another expression.
For instance, if I say “Hello, how are you?” a formulaic response is “I’m fine, how are you?” Another example would be if I say “Good bye” you could say “See you later.”
These expressions are very common and so they are essential to learn. If you are in an beginner or low intermediate course, this activity should be right up your alley. (Ask your teacher what that idiom means!)