When you roll two numbers you have four kinds of outcomes: EE, EO, OE, OO (where E = even, O = odd).
You get an even sum only if you sum two numbers that are both even or both odd, therefore the outcomes wanted are two: EE and OO.
Therefore:
P(sum is even) = 2 / 4 = 0.5
Hence, there is a 50% probability that, when you roll two numbers, their sum will be even.