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.
watch over (someone or something)
to monitor or guard (someone or something)
I watched over the class while the teacher was at a meeting.
List of common idioms, their meanings and examples of their usage in sentences
An idiom is defined as - a group of words whose meaning is different from the meanings of the individual words.