The entire passage you are asking about reads

A technological output and explosion on the farm has led to an output explosion. An urban population revolution has overcrowded our schools and cluttered our cities and crowded our slums.

The major phenomenon that Kennedy is referring to in this passage from the speech is urbanization.  He is talking about a process in which Americans moved to urban areas from rural areas.  This happened, he is arguing, because of improvements in technology.  Technology made farmers able to grow more crops with less labor and so many rural people moved to cities for work.  There, in the cities, these migrants encountered problems of overcrowded schools and slum living conditions.  Kennedy is arguing that his leadership, the leadership of a young man with new ideas, is needed to address this new problem.