In this satirical essay, the speaker wants to figure out how to end the problems of poverty that are being experienced by the people of Ireland during his time.
His modest proposal that he humbly offers is that the Irish (who have, in his opinion, too many children) should sell their children to the rich. The rich can use them for food. That way you get rid of the overpopulation and the poor get some money.
By writing the essay in this way, Swift criticizes both the way that Ireland is run by the British and the way that some people rely on reason rather than having any emotions or sympathy for others.