As you've already noted, a sealed class is actually relatively simple to understand. You can simply mark a class as sealed :
sealed class MyClass
And this just means that no class can then inherit from it :
class MySecondClass : MyClass //Compiler error
But as for the second part of your question, where would you actually use it? If you are looking to really lock down a class and make sure it can't be overriden in any way (So the functionality is always the same), for example if you are building a C# library, you may wish to seal it.
In saying that, if you scroll through the .NET Github repositories, you will find countless examples of users complaining that a particular class is sealed for no good reason and can it be unsealed. If anything, you'll probably run into more cases where you'll be asked to unseal a class than times where you wish you had sealed it.