Iron has the most stable nuclei because of... chemistry reasons... so it was thought most radioactive decay chains would effectively end there. This is also neat on an astrophysics level because iron is the last element created by first generation stars, so you'd get this grand entropic cycle that ends with a universe made of black holes, neutron stars, iron, and loose hydrogen atoms, more or less. In theory but not practice, probably.

For all human practical purposes decay chains end in lead though. The time scale in the meme is the difference between "effectively stable" lead and "as stable as possible" iron.

Tl;Dr everything below iron will get fused into at least iron by stars. Everything above will decay into iron.

Elements with a proton number less than or greater than iron become more stable as they approach iron, and so, over very, very long timeframes, from random quantum tunnelling effects, will favour being iron over being other elements.

Thus, in 10^~1500 years, virtually all atoms in the universe will turn into iron, assuming protons don’t decay (which may or may not happen).

Iron-56 is one of the most stable nuclei, along with nickel-62, so after untold time lengths, due to standard fissile decay/spontaneous fusion (possibly due to quantum tunneling), all baryonic matter decays into iron-56, and a little nickel-62.

To my understanding, elements smaller than iron can produce energy in nuclear fusion reactions (like in stars and such), but bigger elements require a net energy input to make them fuse. Meanwhile, bigger elements eventually decay into smaller ones (though many take an extremely long time). So, given a sufficient not quite eternity of time for everything lighter to get fused together and everything bigger to decay, iron is the midpoint everything ends up as.