cosmology - What is the Greisen-Zatsepin-Kuzmin (GZK) cutoff and why does it happen?

I was just wondering, what is the GZK cutoff point and why does it happen? Some say it is some sort of 'lattice' in the universe; https://www.google.ie/amp/s/io9.gizmodo.com/5950543/physicists-say-there-may-be-a-way-to-prove-that-we-live-in-a-computer-simulation/amp

Is this true?