What do the numbers mean on a grenade mod?
It is the damage but it doesn't necessarily mean that its going to hit for 45 points of damage. Keep in mind the damage that you do depends on the level of the person you are blowing up (in relation to your own level) and the chance for the grenade to "crit".
So throwing a grenade at a person of a lower level compared to you will do a lot more than 45 damage.
Oh its worth mentioning if its a MIRV or "cluster" mod then you're going to see a heck of a lot more than just one big explosion.