NaN stands for Not a Number. to simplify very briefly (and not accurate at all), when defining a standard for representing fractional values using binary digits in computers they systematically assigned natural numbers in a range of values to some fractional numbers. some of the possible natural numbers for reasons not worth talking about were unused, so they were designated as NaNs, and the value of the NaN itself is supposed to tell you what went wrong in your calculations to get a NaN. obviously if you use a NaN in an arithmetic operation the result is also Not a Number and that’s what the meme is referring to.