Below the Lords are the Knights. Each Lord has a Second - that is, someone who rules the land in his stead when he needs to go to council or the Lord is incapacitated, or the Lord wishes to go on vacation. This Second is always a Knight.
Some knights are Swordmasters. All knights are their lord's Champions, and all follow their lord to the letter. The only way a knight can separate himself from his lord is by giving back that lord's colors in a public display of repudiation. Then the man is no longer a knight, but after a year and a day, can be picked up by another lord if the ex-knight so desires. Though he is no longer a knight, he can choose to settle elsewhere in private, or be a King's Blade or a mercenary. It is not surprising if a lord chooses to summon a King's Blade to take out the ex-knight.
A lord gives to the knight land, lodging and money (if necessary) to pay the people to work the knight's land. In return the knight will run parts of the lord's army, train new recruits, advise the lord in all matters public and personal (some knights have a council, others are just advisers of certain matters), guard the lord's life with their own, and be a champion or proxy for him when visiting other lands.
Sometimes a new lord is from the Second, other times it is from the Knights, yet other times it is from the people themselves. In some lands, politicking is frowned upon.