TLDR:: In your specific situation, you just need to set the weight to one for the whichever of the +1 stats you want, and let the "Magic Weight >=" to 1. Since the other stat is fractured you don't need to worry about anything more.
If you want a better idea of what it's doing or how to use it, continue reading/
Weight is how much you value that mod. Specifically, this is important when you want the mod to roll multiple modifiers. If you are crafting something and you want two specific mods (prefix & suffix), you could just give each a weight of 1 (default), then in the previous menu you would give the item a magic or rare (depending on if you are using alts, or chaos) weight of 2. This would mean that the program will continue to reroll the item until the "total rolled weight" is 2 or greater, aka both of the mods you selected.