Let me first cover ARQ and HARQ.
ARQ-
1. In ARQ, if receiver has not received packet correctly. Receiver will send NACK for that packet to transmitter. And whatever bad packet, it has received, receiver will discard packet simply.
2. And in case of successful reception , it will send ACK to transmitter.
While in HARQ
1.In case of unsuccessful reception, At Receiver side,instead of discarding bad packet ,buffer that bad packet .And that packet will be combined with the next re-transmission(i.e., two or more packets received, each one with insufficient information to allow individual decoding can be combined in such a way that the total signal can be decoded!)
So , in case of HARQ, redundancy version comes into the picture.
Let's say, Transmitter has transmitted one packet [5]. And it has not received correctly at receiver side. Then transmitter will again transmit next version of that packet [5.1] which can be identical to the original packet or it can be only redundancy .
And then, if we re-transmit less information (only redundancy), we spend less energy, and that will run much faster. With this we have a gain!
That is, we work with different 'versions of redundancy', that allows us to have a gain in the re-transmission. This is called 'Redundancy Version', or what version of redundancy.
I hope its clear. In case of further doubts let me know.
