serializable is an interface which is present in java.io package and serializable is a marker interface.

Usage:- serializable is an interface which is used to mark a class whose object can be serialized.
seralization refers to the process an object can be converted into a format in which it can easily be stored.
for example:- A file is transmitted over a network

When object need to store persistently like in file system or database serialization allows you to convert object into stream of bytes that can be easily return to disk or transmitted over a network.
 static members cannot be serialized.