If someone is skinnier, they are thinner than they were before or thinner than someone else.
Being skinnier can be good or bad, depending on the situation. It might make someone feel happy or worried.
Examples:
She is skinnier than she was last year.
He is skinnier than his brother.
The skinnier cat got more food.
My dog is skinnier than your dog!
The skinnier you are, the easier it is to run.
I used to be skinnier when I was a baby.