When you engage in two-weapon fighting, you wield your blades in a lethal frenzy. When you take the Attack action and attack with a light melee weapon that you're holding in one hand, you can make another attack with a different light melee weapon that you're holding in the other hand as part of the same action. You don't add your ability modifier to the damage of the extra attack, unless that modifier is negative or you have the Two-Weapon Fighting Fighting Style.
If either weapon has the thrown property, you can throw the weapon, instead of making a melee attack with it. You can still use a bonus action to make an attack with one of your light melee weapons as usual when using two-weapon fighting, and the extra attack granted by this feature is in addition to any other attacks you can make as part of the Attack action, such as those granted by the Extra Attack feature.
You can only benefit from this feature if each weapon you are wielding has the Light property.