A more thorough explanation:

Definition: The Master of the Horse is a peer in English law who is the third officer of the royal household, after the lord steward and lord chamberlain. They attend the sovereign on state occasions. Originally, the official was in charge of the royal stables, but that duty is now entrusted to the Crown Equerry.

Example: During the coronation of a new monarch, the Master of the Horse would ride alongside the royal carriage as it makes its way through the streets.

Explanation: The Master of the Horse is responsible for ensuring that the horses and carriages used by the royal family are in good condition and ready for use on important occasions. During a coronation, the Master of the Horse would ride alongside the royal carriage to ensure that everything goes smoothly and that the horses are well-behaved.