Marking a type as sealed means that it can only have direct subtypes that are declared in the same source file.
For example, declaring this in, say, Foo.scala, will compile:
sealed class Foo
class Bar extends Foo
But the following, in Baz.scala, will not:
class Baz extends Foo
This is useful when all subtypes of a given type are known and we want to let the compiler know about it - typically, when working with ADTs.