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is this in English ?
Fustian is heavy twilled fabric, more or less like blue jeans. A jupon is a long coat, and a habergeon is a sleeveless coat of mail. Here we're told essentially that he's wearing plain jeans and plain coat, that's been stained by his chain-mail (I guess with rust).
So he's dressed plainly, and his clothes are designed to be simple and long-wearing, and yet are nevertheless worn with his travels (detailed in the lines above), so we it seems he is, despite his high rank (of knight) a humble man, not given to showy dressing, and moreover one who takes his vocation (being a knight) seriously, and bears the marks of wear of a serious fighter (the wear on his clothes).
Presumably Chaucer is trying to contrast him with a knight who uses his high position to wear expensive, flashy clothing, and who takes such care of his person and his clothes that they are never stained or worn -- who doesn't take his responsibilities as a knight seriously.