X needs to be multiplied by the same number each time to get Y. Each time you have to multiply X to get Y, the number you have to multiply by is different. If the relationship was proportional, you could take one number and multiply it by X to get Y.
Ex.
X= 2, 6, 8, 12, 18
Y= 1, 3, 4, 6, 9
The rule there is that you have to multiply X by 2 to get Y.
Hope this helps!