This is because a metaphor is something that is being compared to another thing in the form of saying that it is (although with context you can see it is).
An example of a metaphor is “The plant was a ferocious beast.” Obviously, the plant isn’t actually a ferocious beast, but we can infer that the plant was overgrown and consuming the walls.
As opposed to a simile, which is basically the same thing as a metaphor, but using ‘like’ or ‘as’.
An example of a metaphor is “The plant was like a ferocious beast.”
