Chivalry, which comes from the French word for "horse," is essentially a code that emerged in the Medieval world that governed the behavior of knights toward not only other knights, but women, rulers, and everyday folks.
And while chivalry was certainly a concept with some currency in the real Medieval world, chivalry most flourished in the imagination of poets and writers who used it as a template to envision a quasi-historical, quasi-invented medieval world where knights wandered from kingdom to kingdom helping those in need, destroying those who did wrong, and falling madly, and unwaveringly, in love with deserving ladies.
Chivalry is an ideal that continues to inspire fiction and non-fiction today, from the perpetual popularity of Disney princes and princesses to contemporary romantic comedies. To call it unrealistic is a massive, massive understatement.
So while Spenser certainly isn't in a minority for finding this concept creatively generative, it's worth examining why he turned to this particular code since it had essentially died out by his own time, the Renaissance. Knights had been replaced with courtiers, who more frequently dueled with their wit and political savvy than they did with actual swords. We certainly see evidence in the poem that Spenser was partly trying to critique the courtly world that had largely replaced the chivalric one, and Spenser may be engaged in a bit of nostalgia for a code of behavior that had now vanished… if it ever actually existed.
Spenser is also quite directly exploring the courtly realities of his own time through this out-of-date theme. We can also see a connection between the old values of chivalry and the new values of the court. In particular, the courteous, but respectful, courtship of women remained at the center of both behavioral codes, and was a particularly delicate issue since England was at this time ruled by a woman, Queen Elizabeth I.
Queen Lizzy organized her court around the principle that her trusted and loyal advisors behaved towards her as if they were courting her, and got super-furious if she discovered that any of them had other affections. Whoa. Simmer down, Elizabeth.
We can also see Spenser critiquing chivalric ideals, since so many of the knights we come across in the poem don't exhibit chivalric codes of conduct at all: they injure the helpless, act selfishly, and are obsessed with power. Clearly, the idea of chivalry isn't enough to ensure that people behave morally to one another. What's needed? Christian morality, another crucial code of behavioral conduct we see explored and represented throughout the poem.