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To extend others' answers, a sealed class (the equivalent of a final class in Java) cannot be extended. This means that any time the compiler sees a method of this class be used, the compiler knows absolutely that no runtime dispatching is needed. It does not have to examine the class to see dynamically which method of which class in the hierarchy needs to be called. This means that the branch can be compiled in rather than being dynamic.
For example, if I have a non-sealed class Animal that has a method makeNoise(), the compiler does not necessarily know whether or not any Animal instance overrides that method. Thus, each time any Animal instance invokes makeNoise(), the class hierarchy of the instance needs to be checked to see if the instance overrides this method in an extending class.
However, if I have a sealed class AnimalFeeder that has a method feedAnimal(), then the compiler knows with certainty that this method cannot be overridden. It can compile in a branch to subroutine or equivalent instruction rather than using a virtual dispatch table.
Note: You can use sealed on a class to prevent any inheritance from that class, and you can use sealed on a method that was declared virtual in a base class to prevent further overriding of that method.