The roughest thing is the most not smooth thing.
Rough things can feel bad or be hard to use.
Examples:
The rock is the roughest thing I've ever felt.
The roughest part of the road is where it's all bumpy.
She has the roughest hands.
The roughest blanket is the one made of rocks!
The roughest sandpaper can scratch anything.
The roughest part of the mountain is the very top.