Metaphor is often compared to simile, which is when something is compared to something else.
Example
'I have a lot on my plate', 'Sales really took off' and 'He's a real couch potato' are all metaphors.
In the classroom
Metaphors are a rich and enjoyable area of language for learners but also can be problematic, as their meaning is not immediately obvious. Activities that encourage the use and study of metaphors include; creative writing, classroom posters of metaphor groups, looking at euphemisms and finding L1 equivalents.
Further links:
https://www.teachingenglish.org.uk/article/marvellous-metaphors
https://www.teachingenglish.org.uk/article/exploring-metaphors-classroom