Affinity in Monster Hunter World (MHW) is the weapon's chance to deal more or less damage on attacks. Affinity is determined by the weapon and skills that are being used. This works on a percentage so for example at -100% you will always deal less damage. At 100% you will always do more damage. Some skills will increase your weapon affinity OR with the Crit Draw skill you will always Crit on a draw attack. Attacks that crit deal +25% more damage, while the opposite is -25%. Successful critical hits are signaled by a flashy slash animation and a specific sound effect. Hell yeaaaah!!
Skills that Increase Weapon Affinity