What does this mean?yet what I sowed and what the orchard yields my brother's sons are gathering stalk and root: small wonder then my childeren glean in fields they have not sown, and feed on bitter fruit.

Line Six: It expands on the point made in line five.
Line Seven and Eight: The poet discusses the reaping announced in its title. Reaping of grain is generally done with a scythe (a farming tool with a long cured blade) or machine, cutting down wide columns of grain stalks with each pass.
LIne Nine: The speaker's work ethic is on display, as he talks about the balamce between what he has sown in the field and what fruits the field has borne. Although the speaker does not derive that much benefit from his work, the poet's wording in line nine betrays a pride for what little he has gained.
Line Ten: Refers to extended relations, not his direct descendants, and so readers can assume that "brother" is meant in the broadest sense, as as reference to all humanity.
Line Eleven: To "glean" means literally to gather what is leff on the ground after reapers have taken away the important parts of the harvest.
Line Twelve: The up-and-coming generations of black Americans, the speaker says, will have to fend for themsleves. The fields that they do not own and have not cultivated are symbolic of the way that black Americans were denied property ownership in the past.