IPA: /ˈsmuːðli/
KK: /smuðli/
In a way that is even and without bumps or interruptions.
The car glided smoothly over the road.
Smoothly → It is formed from "smooth" (from Old English *smoð*, meaning even or level) and the suffix "-ly" (meaning in a manner of). The word "smoothly" means in a manner that is even or level, without roughness or interruption.
Think of something being even and level ('smooth') and done 'in a manner' ('-ly'). This helps you remember that 'smoothly' describes how something is done in a smooth way.