1. To provide lodging and care for a horse: This is the most common meaning. It refers to a stable or boarding facility that provides food, shelter, and care for horses.
Example: "I need to find a place to board my horse while I'm on vacation."
2. To mount a horse: This is a less common usage, but it can be used to describe the action of getting on a horse. It's more common to say "mount" or "get on" a horse.
Example: "He boarded the horse and rode off into the sunset." (This usage is less common and might be considered archaic or poetic.)
It's important to consider the context to understand which meaning of "board a horse" is intended.