"Functional language" is language
that we use to perform various "functions" such as giving advice or apologizing.
Functional language typically uses fixed expressions for each function –for
example "if I were you" or "my suggestion is" in giving
advice, and "it was my fault" or "please forgive me" in
apologizing.
These pages list common expressions for
each function, graded by level. Do have a look at upper-intermediate and advanced levels! (From EnglishClub)