Abstract classes can;t be instantiated, no - that's the whole point. They are a "generic base class" from which concrete examples are derived.
For example, a Car class would be abstract, and give rise to derived concrete classes FordFiesta1300, MercedesA180CDi, ToyotaHiLux, and so on - because you don't buy "a car", you buy "a Ford Fiesta 1300", or "a Mercedes A250e", or ...
Car describes all the thigs that make it "a car" : engine, steering wheel, drivers seat, four wheels, and so on; while MercedesA180CDi fills in the details: "2L Turbo Diesel", "W169 body shape", and so on. The actual instance of a car "my car" adds teh details which differenciate it from all other A180CDis: colour, VIN number, registration number, registered owner, current location, current speed, fuel in tank level, and so on.
In your example:
AbsBase obj =new DerivedBase();
is of the type AbsBase, which means it can contain an instance of any class derived from AbsBase.
In Car terms:
Car myCar = new MercedesA180CDi(Color.Black, "WDD169000J000000, "XX 99 XXX", originalGriff);
can hold any type of Car - a ford, a Mercedes, a Citroen, ... because all such vehicles are derived from the abstract base class Car.
The system sorts out from the individual instance which method it shoudl call when you try to access a base class method: