Metaphors have two parts: tenor and vehicle.
The tenor is the the subject of the metaphor.
The vehicle is the thing the subject is compared to.
Take a look at the image below for a couple examples.
Similes are metaphors that use like or as between the tenor and the vehicle. For example:
My heart is like a singing bird
In this simile, Christina Rosetti compares a heart to a singing bird. Heart is the tenor, a singing bird is the vehicle. She could have written, My heart is a singing bird, but she didn’t take it that far. That’s why metaphors are more powerful than similes, which only declare a thing is like another, not the same.
Other types of metaphors are (definitions from Google search):
Metonymy – the substitution of the name of an attribute or adjunct for that of the thing meant, for example suit for business executive, or the track for horse racing.
Synechdoche – a figure of speech in which a part is made to represent the whole or vice versa, as in Cleveland won by six runs (meaning “Cleveland’s baseball team”).
Personification – the attribution of a personal nature or human characteristics to something nonhuman
Who cares, right? Well, if you’re trying to fix a mixed metaphor, it helps to know which part seems off. Does the problem lie with the tenor or is it the vehicle?