When someone smooths something, they make it flat or even. It can also mean to make something less rough or bumpy.
If someone smooths something, it is usually to make it better or nicer.
Examples:
She smooths the bedsheet before making the bed.
He smooths the sand with his hand.
I smooth the frosting on the cake.
The cat smooths its fur with its tongue.
He smooths the road so it's not bumpy at all.
She smooths the paper with her hand to make it flat.