In your first action your blade starts drawing in air and energy.

In your second action you release this energy with a full swing of your sword. The energy combusts into flames and travels 60 ft. in an arc shape of 15 ft. wide.

Characters hit take 7D8 Fire and Slashing dmg

It ignites flammable objects in the area

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the damage increases by 1D8 for each slot level above 3rd.