This type of plot is common in papers describing theoretical models of supernovae and other stellar explosions. It is a graphical representation of the nucleosynthesis which occurs in the ejected material. This specific plot is from Shen et al. (2010) and represents the detonation of a 0.2 solar mass helium shell on the surface of a 0.6 solar mass white dwarf.
The x-axis is presented in terms of the “mass enclosed”. This is a proxy for radius and is precisely what it sounds like: the amount of mass which is inside a certain location. For this particular model 0.6 is base of the (initially) helium shell and 0.8 is the outer edge. The y-axis is the mass fraction, with 1.0 representing all of the mass at a certain radius. Each colored line therefore tells you what fraction of the mass at a certain radius is made up of that element. For example, at a mass enclosed of ~0.66 Msun, 80% of the material has been burned to Nickel (black line).
You can see in this particular model, most of the inner part of the shell was burned to iron peak elements (titanium, chromium , iron, nickel), while in the outer part of the shell over half of the mass is still unburned helium.