Idioms are phrases which cannot be understood simply by looking at the meaning of the individual words in the phrase.
say the word
to give a sign, to show a wish
"Just say the word and I will come and meet you at the airport."
List of common idioms, their meanings and examples of their usage in sentences
An idiom is a group of words, or in other words, a phrase that has a meaning different from the literal meaning of the words in it.