In D&D 5e two weapon fighting is a bonus action that can be taken in combat.
For reference...
Basic Rules:
https://www.dndbeyond.com/sources/basic-rules/combat#Two-Weapon-FightingWhen you take the Attack action and attack with a light melee weapon that you're holding in one hand, you can use a bonus action to attack with a different light melee weapon that you're holding in the other hand. You don't add your ability modifier to the damage of the bonus attack, unless that modifier is negative.
If either weapon has the thrown property, you can throw the weapon, instead of making a melee attack with it.
NOTE:
- D&D 5e does not have main hand and off hand distinction. Either weapon can be used for action / bonus action attack. Subtle but important for those who wield two different weapons on turns they wish to use only their "off-hand" weapon and a different use of their bonus action.
- BG3 does not require attack action to use two-weapon fighting.
- Two weapon fighting style (Fighter, Ranger) allows to add modifier to two weapon fighting attack
- Dual Wielder feat allows non-light weapons.
