What is marker interface?

Answer 1

an interface that doesn't have anything in its block.
it is not implemented to give classes and their objects any type of action by defining any method or declaring it and enforcing implementing-classes to override it. it is just used at compile-time to ask JVM, check some behavioural features of a class that implements it.
they are like annotations but have their differences

for better understanding check this link:
https://dzone.com/articles/are-marker-interfaces-dead