I don't understand constants

sodium.dioxid

What exactly are constants? Are they just used to make the equation work or is there something more meaningful than this? Right now I am studying rate of reactions and they go something like this:

rate = k [A]^x [B]^y

It seems like k is only there to make the equation work. It would seem like k is the responsiveness of rate to concentration. But this becomes meaningless when comparing two reaction of different orders; they take on different units for different orders.

Drakkith

Have you read this article? http://en.wikipedia.org/wiki/Physical_constant