You brandish the weapon used in the spell’s casting and make a melee attack with it against one creature within 5 feet of you.
On a hit the lightning magic embued into the weapon will erupt in a small lightning nova around the target hit. 

The nova deals 1d6 Lightning Damage to the main target, and damage equal to your spellcasting modifier to any enemy within 5ft of your main target.

The damage of both attacks increases by 1d6 at levels 5, 11 and 17