It is basically a way to put ownership on something that can be replicated.

Mostly popular for pictures

Let's say you are an artist and want to make a digital picture. There is nothing on that picture to say it is specifically yours. And transfering ownership would be you just sending them the picture. However if it is an nft it will say who the owner is. And if you transfer ownership to someone else, then they are now the owner.

In theory you could compare it to a deed for a house. Only difference is that the house would be digital. The ownership is held by a wallet and not a person.

It is simply making something that is untangible into something tangible, if that makes sense.

It can have a lot of cool uses. Lets say you go to a digital art creator's physical store and purchase a digital picture with physical money. The artist could cive you a key containg the wallet that had the ownership for that picture, and since you now have the wallet, you are now the new owner of the picture. You have made something that is normally not a physical item, into something that is physical, and therefore can trade it like you would with a normal painting.