It's about the feasibility of breaking 256-bit encryption keys with a naive brute force attack. The simplest component of a computer that performs this attack would be a 256-bit counter that iterates through all possible key values, however Schneier is saying that according to our current understanding of physics, even if you built the most efficient computer possible and powered it with the energy output of a supernova, it would only have enough energy to iterate through all the values of a 219-bit counter.