An idiom is an expression where the meaning is different from the meaning of the individual words.For example, to have your feet on the ground is an idiom meaning ‘to be sensible’.
A lot of idioms are formed using phrasal verbs.
For example, after he left me, it took me a long time to pick up the pieces.(= return to a normal life.)
Many idioms are colloquial,which means that they are used in informal conversation rather than in writing or formal language.
For example, I won’t tell anyone your secret.My lips are sealed.