The constant of variation is basically a unit rate in a proportional relationship.
For example, 1 apple equals $3.00
So, the x would be the amount of apples since it's a quantity or term. And y would be the money.
So you could write the proportional relationship like this y = kx
k stands for the constant of variation. So in this example, it would be like this y = 3.00x
So let's say Milly buys 3 apples, we could write that proportional relationship like this y = 3.00(3) which means that y = 9.00 Since Milly bought 3 apples
I know this was not the best explanation. But I hope that you got a better understanding of what constant of variation means ^^