If you need a laymans explanation of the use beyond that provided in the Spring Docs
Consider this code...
class Service {
@Transactional(propagation=Propagation.REQUIRED)
public void doSomething() {
// access a database using a DAO
}
}
When doSomething() is called it knows it has to start a Transaction on the database before executing. If the caller of this method has already started a Transaction then this method will use that same physical Transaction on the current database connection.
This @Transactional annotation provides a means of telling your code when it executes that it must have a Transaction. It will not run without one, so you can make this assumption in your code that you wont be left with incomplete data in your database, or have to clean something up if an exception occurs.
Transaction management is a fairly complicated subject so hopefully this simplified answer is helpful