It's one of those in-jokes that someone with A level Maths might find funny.
Square root of -1 is given the symbol i.
(If square root of -1 = i, then i2 = -1. When you square a positive number like +3, you get +9. When you square a negative number like -3, you also get a positive number 9. Hence, it appears to be impossible to have i2 = -1. Hence i is called an imaginary number.)
23 = 2x2x2 = 8 which sounds like ate.
The symbol Sigma in Maths means the SUM of
The pi symbol is well-known.
Hence "I ate some pie"