It is a modest proposal to do about the problems in Ireland. Swift wrote this pamphlet in 1729 right after James II abdicated the throne of England.
Swift gives a whole range of satirical solutions involving letting the Irish starve or letting the Irish use each other as food in the form of cannabilism by fattening up the children and selling them on the meat markets.
I have been assured by a very knowing American of my acquaintance in London, that a young healthy child well nursed is at a year old a most delicious, nourishing, and wholesome food, whether stewed, roasted, baked, or boiled; and I make no doubt that it will equally serve in a fricassee or a ragout.
Swift's "modest proposal" is ironic and satirical meant to point out the unfair and hypocritic way that the British government had dealt with the famine in Ireland.
Swift is pointing out that the British landlords are the ones who are causing a lot of the problems that are forcing the people out of their homes.
I grant this food will be somewhat dear, and therefore very proper for landlords, who, as they have already devoured most of the parents, seem to have the best title to the children.
Swift is ridiculing the Irish for their Roman Catholic religious practice which encourages childbearing. However, he is also ridiculing the British landlords for their iron fisted greed regarding rents and taxes.
Secondly, The poorer tenants will have something valuable of their own, which by law may be made liable to distress and help to pay their landlord's rent, their corn and cattle being already seized, and money a thing unknown.
By saying that the Irish sell their children for meat, he is leveling the harshest criticism against the British government for allowing the situation in Ireland to become so dire that such a thing might even be considered even in satire.
The irony here is that Swift's use of the word modest is a gross exaggeration. There is nothing "modest" about Swift's proposal in dealing with the Irish problem.