This phrase is a simile, since it uses as and compares two things to each other:
The butterflies drifted as lazily as falling leaves.
It's comparing how butterflies fly to leaves falling. Generally when you think about leaves falling, they are floating gently and delicately. It doesn't occur quickly, but instead in sort of slow motion and light-weight manner. There is no hurry or rush. Wherever the wind blows the leaf goes. Since the butterflies are being compared to the falling leaves we can assume that the butterflies have these gentle and slow qualities like the leaf.
I hope this helped! Let me know if there is anything else I can do!