When people talk, their voices go up and down in pitch and loudness, kind of like a rollercoaster. Smoothing in phonetics is like taking that rollercoaster and making it smoother so it's not so bumpy.
Think of it like when you draw a line with a crayon. If you draw really fast, the line might be bumpy and uneven. But if you draw slowly and carefully, the line will be smoother.
In phonetics, smoothing is when someone tries to talk more evenly without going up and down too much. It makes their voice sound smoother and easier to listen to, like when you're listening to a bedtime story.
Sometimes people need to do smoothing if they have a speech disorder or if they're learning a new language. It's like practicing drawing slowly and carefully so they can get better at it.