From the post:
Unlike items with the invested trait or ones that you activate, weapons typically require no Resonance to use. You just pick one up and swing (or shoot, or sling, or thrust, or throw), and the magic weapon unleashes its punch, often with potency. Think as a weapon's potency as its "plus" and more. Potency still grants an item bonus to your attack rolls with the weapon, but now it also increases the damage dealt by an additional weapon die for each point of potency bonus. For example, let's say you find a +1 longsword buried in an otyugh's dung heap. Regardless of its current soiling, that weapon grants not only a +1 item bonus to attack rolls, but on a successful hit it deals 2d8 + Strength modifier damage, instead of the standard 1d8 + Strength modifier. A +2 longsword would instead grant a +2 item bonus to attacks and deal 3d8 + Strength modifier damage, and so on.