Common Expressions
Please excuse me.
This sentence is a COMMON EXPRESSION. Any group of words that describe an idea is a phrase, and when a phrase is widely used, it becomes a way of speaking, and this is what an expression is.
The difference between an expression and an idiom is that the words in an expression make the meaning of the expression quite clear. The words in an idiom do not make the meaning clear. It is impossible to work out what an idiom means by analysing the words.
Common expressions and idioms are an essential part of English vocabulary, and appear regularly in CAE sentence transformations.