The graphic is an attempt to project the 3D observable universe down to 2D and then depict the time evolution of it to show that it has expanded in spatial volu

The graphic is an attempt to project the 3D observable universe down to 2D and then depict the time evolution of it to show that it has expanded in spatial volume, with the bell shape showing extremely rapid expansion during cosmic inflation, then slowing expansion, then accelerating expansion again as matter density became low enough for dark energy density to take over. But the observable universe is not the entire universe. There is no known boundary to space and there probably is no boundary at all. We'll never have any way of knowing, but space could easily be infinite in extent, while still expanding at every point. If you were on the boundary depicted (the cosmic horizon of the Earth-centered observable universe), or even if you were outside of that completely, the picture from your vantage point should look exactly the same. As for what the horizon represents, it's all light that could have reached Earth within 13.7 billion years, that is, Earth's past light cone. It isn't necessarily the case that nothing leaving Earth today could ever get past that horizon. Light can travel forever if nothing ever scatters it. Given some very large amount of time, it can beyond the boundary depicted there. Because of the expansion of space, however, that time is likely to be more than 13.7 billion years, and given the acclerating expansion of space, there will eventually come a time that no light leaving any point within our local supercluster can ever reach any other supercluster. They will become forever outside of each other's horizons.