It suggests a satirical and simple proposal to the problem starving Ireland faced: the lack of food to feed the people.
Of course, the entire modest proposal was that parents eat and sell their children as food and the idea was meant as satire, meaning to be considered with a grain of salt and not seriously (kind of like a really dark joke).
''Modest'' means that the proposal was obvious and simple in the eyes of the author Jonathan Swift but not necessarily in the eyes of others. ''Proposal'' means the general answer to the problem Swift was answering.
