Swaps your physical attack and magic attack power values.Description (misleading)
ATK⇔MATK materia is a 
Complete Materia in Final Fantasy VII Rebirth that switches the user's 
Strength attribute with their 
Magic (and not their attack power and magic attack like the description claims).
Obtained
A ATK⇔MATK materia is found in the Research Facility E4 in the Shinra Manor; the player needs to move through the vents that only Cait Sith can navigate.
Use
ATK⇔MATK swaps the party member's strength attribute with their magic attribute. As such, this can give a party member with normally high magic attack attributes (such as Aerith) a high physical attack potential, or one with normally high physical attack attributes (such as Tifa) a high magic attack potential. The materia only swaps the base stats, so the player cannot equip a magic-focused weapon with the hopes it will give good physical damage with ATK⇔MATK. However, boosts from accessories and stat-enhancing materia are counted.
A likely better way to raise attacking power is with equipment and with Strength Up Materia and Magic Up Materia, and with the buffs 
Berserk, 
Bravery, and 
Faith. However, ATK⇔MATK can be good for a magic-focused Tifa, as her ATB fills faster and so she can potentially be a more efficient spellcaster than Aerith, and can also use Starshower to power up her follow-up spell. Her Unfettered Fury also turns her unique abilities into magic attacks, and she also has Chi Trap. However, even Tifa, who has the biggest difference between strength and magic stats to strength's favor, has a difference of less than 20 at maximum level. The biggest difference is for Aerith, but there is little point to boosting her strength stat as her attacks are magical anyway unless she is transformed into a frog.