Not, in this case, a mis-spelling of Cain, but rather a Scottish term, meaning payment in kind - a form of rent paid by tenant farmers to the land's owner, usually in the form of produce, wheat, and other goods.
More generally, kain - or cain - is services or goods rendered on a regular basis in return for some action or debt. Usually this is rent of some sort, but can be nearly any service. The distinction is that cain is not paid in coin, nor is it barter per se - barter connotes some one-time deal or action, while cain suggests a continuing - if perhaps informal - agreement.
Although uncommon in most cities and large towns, or in areas with well-developed and formal systems of exchange (which tend towards various forms of currency as arbiters of value), kain is far from uncommon in smaller villages, hommlets, etc. where coin may be sparse (and reserved for trade with wandering merchants, etc.) and reputations better known.
Knights, Orders of
Although few nations support large standing armies, it sometimes seems like you can't swing a dead goblin without hitting some new Knightly Order raised by one nation or another. There are a number of reasons for this - Knights are required (in general) to raise and support their own troops (if only a handful - but even the simple maintenance costs for a squire or two, 2-5 pages, and a dozen men-at-arms, plus horses, armor, weapons, etc. can still be substantial), and ties those knighted to the ruler who has granted the title in some semi-permanent fashion (although some Orders have long-since outlived their originators, and are to a greater or lesser extent self-sustaining).
There are many reasons why one would accept knighthood, given the costs (both social and economic) involved - above and beyond any questions of cultural conditioning, sense of honor, ego, and the like. Knighthood is generally seen as (and frequently is) a step up into the nobility (if any) of a state or culture, usually comes with some (minor) grant of land (which can offset or eliminate the costs incurred - and in some cases, even provide a tidy profit), and Knights are granted some measure of immunity, in many cases, to lesser crimes and misdemeanors - particularly involving the lands they have been granted fiefdom over, although this by no means excuses their every action (theoretically, at least).
Although knights are frequently Fighters of some sort, not all of them are. More than a handful of wizards, sorcerers, illusionists, and various other arcane spellcasters have been inducted into knightly Orders, and there are Orders that encourage - or even mandate - some manner of arcane ability. In similar fashion, the more devout Orders can count among their number not only Paladins, but Clerics as well.
Kibble, in this case, refers to the miscellany found in any dungeon. It's all the stuff that doesn't precisely count as treasure - candlestick holders, interesting crystals or river-polished stones used to hold down piles of paper, writing sets, wax sealing kits (for scrolls and letters - although the signet ring used to imprint the letter or scroll might well be valuable), trashy fiction novels, half-finished manuscripts for plays, forks, plates, eating knives and the like, serving platters, tchotchkes, and the like.
Kibble is generally - although not exclusively - valueless to anyone other than the people who accumulated it - Kazorg's collection of knucklebones is unlikely to be of much use to anyone but Kazorg, and nobody wants to know what Mulgrag keeps in that book of his, given that he can't read, but still giggles maniacally as he flips through the pages. Instead, it is useful for building character - players might come to know that an archmage really likes tiny carvings of unicorns, and seek to set him up by creating a situation in which some become available, for example.