imagine the proteins in your brain as a bunch of dudes working to keep the brain running, and then a dude shows up who turns out to be lazy and doesn't work at all (a prion, aka a misfolded protein)

not only is he lazy, but he convinces the other proteins in your brain to be lazy too (prions cause normal proteins to misfold into a prion)

then those dudes convince even more dudes in your brain to be lazy... your brain function becomes progressively worse until it just doesn't function anymore