Answer:
They were the first painters who tried to show three-dimensionality in their works. They did it intuitively observing nature. They manage to have this effect with the method of locating a vanishing point where the parallel lines of an object converged. To give the illusion of depth they painted the figures they wanted to show as farther with a smaller size.
One example is the fresco entitled "The Annunciation" (in Italian, Annunciazione), by the Italian painter Fra Angelico. This painting is very famous because it shows the moment in which Mary accepts to be the mother of the son of God. It shows columns that give the feeling of depth and perspective.
Another example is the fresco entitled "Holy Trinity" by the Italian Renaissance painter Masaccio. This painting is inspired by the Roman triumphal arches and locates the vanishing point at the level of who observes the work. It includes a barrel vault painted in perspective and creates the optical illusion in the viewer who thinks he sees a hole in the wall.