Fortify was mainly used defensively to prevent turrets from taking damage and clearing minions faster than normally. However it was also a very effective method to punish turret divers or quickly do damage to foes that got caught by pulls and taunts like Rocket GrabRocket Grab and Puncturing TauntPuncturing Taunt.
While fortified, a turret grants you assists and kills when it last-hits or assists in killing an enemy champion as if you were dealing the damage, and not the turret. This does not, however, apply to minion kills.
After the V18.104.22.168 patch update, players who achieved summoner level 4 reported having the message that they have learned Fortify. This tooltip may have been left out during the patch's installation.
Reinforce (mastery) – While FortifyFortify is able to be activated, increases damage to minions and monsters by 6. In addition, increases the armor of turrets for 10 seconds after the invulnerability fades.