The Squire is the Knight's son, accompanying him on this pilgrimage. We think he's a pretty good squire; after all, Chaucer tells us that he rides a horse well, can joust well, and he carves the meat for the Knight well at dinner. After the Franklin interrupts his tale, he praises the Squire for being everything a young man ought to be.
But, from his portrait we get the impression that the Squire is still pretty young – more of a man-boy than a man. Chaucer describes him as "embrouded" as if he were a "meede / al ful of fresshe floures, whyte and reede" (89 – 90) – embroidered like a meadow full of red and white flowers. That sounds more like a description of a maiden than a man! And, in his portrait, Chaucer spends a lot more time talking about how well the Squire can dance, sing, write poetry and, most importantly, indulge in serious crushes, than he does about the Squire's prowess on the battlefield. From his portrait, we end with the impression that the Squire is a teenager, a bit of a pretty boy, and prone to serious infatuations that keep him up all night.
The Squire's role in the General Prologue is probably to represent both youthfulness and femininity. Compare his portrait to that of his father, who is a grown-up version of the Squire, or to the Monk, who is a manly man, and we can get a good picture of how Chaucer was thinking about the differences between youth and age, and femininity and masculinity, and how these different categories are all related to each other.