Squire

Bastian Silverhair: "Do you even know what a squire is?"
Gilbert Rivers: "An attendant to a knight?"
Bastian Silverhair: "I'm not a knight. That means you're not a squire!"
— Bastian Silverhair to Gilbert Rivers.
A Squire is traditionally a young man that enters the service of a knight or lord usually with the promise of attaining knighthood at some point in the future, when the squire's master deems it proper. Squires usually take the role of a personal servant, running errands for their masters and keeping their armor, shield, weaponry, and horse ready for battle, tourneys, or tournaments.
Men who don't have the money to buy armaments or the ambition to become a knight stay squires, and become men-at-arms. Squires often fight by their master's side in battle, though very young squires are not normally allowed to participate in combat.
Even a knight can serve as a squire for another man, normally a lord. Ser Yunus Sand squired for Lord Laenor of Pyne during the trial by combat in Castle Liverpool.