If something is the nicest, it is the most good or the most pleasing out of all similar things.
The word nicest makes people think of good feelings and happiness. It is a very good thing.
Examples:
This is the nicest cake I have ever eaten.
She is the nicest person in the world.
The nicest part of the day is when the sun goes down.
The nicest thing about my dog is that he always makes me happy.
The nicest beach in the world has the softest sand and the bluest water.
The nicest surprise is when you find something you thought was lost.