Among Chaucer's pilgrims on the way to Canterbury are two brothers, the Parson and the Plowman, and they are described together not just because of their relationship but also because they are both good men.
Two of the virtues by which medieval men were measured were a) how well they carried out their primary role in life and b) how close they came to the Christian ideal. The first observation Chaucer makes about the Plowman is
A trewe swynkere and good was he,/Lyvynge in pees and parfit charitee./God loved he best with all his herte. . . (ll. 531-533)
A great worker was he/Living in peace and perfect charity [with his fellow men]. He loved God with his whole heart more than anything else.
In a few words, Chaucer tells us that the Plowman is not only truly religious but that he lives peacefully with his fellow men. More important, however, the Plowman exhibits true Christian charity--he is willing to thresh wheat and dig ditches without any payment for any "povre wight" (poor man) who needs his help. Like his brother, the Parson, who is described as leading his parishioners "by good ensample [example]," the Plowman is a model Christian.
It is quite likely--and many scholars have pointed this out--that Chaucer's description of the Plowman is a reference to William Langland's The Vision of William Concerning Piers the Plowman (ca. 1362), a work in which a plowman embodies the ideals of Christianity.