@GunHusky each percent of weakness is 1% less in your stats after all other calculations. E.g., you have 500 health and 100 all other stats. 1% weakness would make that 495 health, and 99 all other stats. 50% weakness would be 250 health and 50 all other stats.
For anyone who has been playing this game for a while and is trying to figure out how the new forge works, you have to turn tutorials on first to unlock it.