A metaphor describes something as if it was something else.
For example:
This is a metaphor because it means 'my feet are very cold', but doesn't literally mean that the speaker's feet are blocks of ice.
If the speaker had said, 'My feet are like blocks of ice,' or 'My feet are as cold as blocks of ice,' it would have been a simile rather than a metaphor. Similes compare things by using the words 'like' or 'as....as'.