Look at the figures, left-to-right. First, there's one supercritical phase - one single phase above the critical point - everywhere in the jar. Right above criticality, the jar becomes clouded over everywhere - this is critical opalescence. Slightly below the critical point, the carbon dioxide is still cloudy. Finally, sufficiently below the critical point, the carbon dioxide is no longer opalescent.
You might be wondering what the three balls are. These are to help us see the density in the jar. Each ball is at a slightly different density - one has the same density as the carbon dioxide at the temperature and pressure shown in the left-most picture, one is a little below, and the last a little above. In the left-most picture - below criticality - the ball with the same density as the carbon dioxide is floating in the middle of the container (neutral buoyancy). The denser ball has sunk to the bottom, and the less-dense ball is floating all the way at the top. In the next picture, the densest ball is still at the bottom, and the least dense still at the top, but the remaining ball has sunk to the bottom. The reason that it has fallen has to do with fluctuations in the CO2, and will become clearer later. In the third picture, something special has happened - the least dense ball has sunk to the middle of the container, where it is joined by the ball with middle density. In fact, these two balls are floating on top of a denser liquid-like carbon dioxide - the phases are beginning to separate! Finally, all three balls are found at the center, floating on top of the denser liquid phase just like boats at sea. Above the level of the balls is the less-dense gaseous carbon dioxide.