The string is an identifier; it's used to make it easy to turn various factors on and off (since you can remove a modifier by id instead of needing to figure out the exact counter-modifier to add); it's also used to prevent modifiers from stacking up infinitely - i.e., if you call modifyMult( "someID", .5 ) on every frame, you get a modification of 1/2, not 1/2 the first frame, 1/4 the second, 1/8 the third, etc.
So basically you just need to use a reasonably-unique non-empty string. I'd suggest just using the name of your mod.