Literally, a "dent" is a small depression in a piece of wood or metal, made by something hitting it.
Imagine a car moving slowly in a parking lot and hitting another car. It doesn't do any serious damage. It just puts a dent in the metal. Old cars gradually get scratches and dents. Big hailstorms put dents in cars. Grandchildren playing with toys sometimes put small dents into our wooden furniture.
So, figuratively, a "dent" means "damage that is ugly, but not serious."
Let's say Mars sold $30 billion in the year 2017. Let's say Hershey, a big competitor, introduces a new candy bar (for example, Hershey's Gold, a bizarre product that contains no chocolate).
If, as a result of Hershey's new product, Mars only sold $29 billion in 2018, we might say "Hershey definitely put a dent in Mars' sales this year."
If Mars sold $30 billion, we might say "Hershey didn't even put a dent in Mars' sales."
If Mars only sold $20 billion, we might say "Hershey did serious damage to Mars' sales this year."