Here's how it breaks down:
* "High horse" is the metaphor. It refers to someone acting as if they are morally or intellectually superior to others, like they are sitting on a high horse looking down at everyone else.
* "Get off" means to stop acting in this superior way and come down to the same level as everyone else.
So, the literal meaning of the idiom is impossible to understand as it is not meant to be taken literally. It's meant to be a figurative expression used to tell someone to stop being arrogant or condescending.