The Kokish Relay uses the 2H rebid to show a balanced hand with 23-24 HCP in this auction:

2*	:	2^
2

* Strong and artificial, a la Standard American
^ 0-7 points or waiting

2 is a relay to 2, over which opener rebids 2NT. An immediate 2NT rebid by opener over 2 shows a balanced hand with 20-22 HCP.

An opening 2NT bid is then freed up to take on whatever (legal) meaning a partnership wants.

Because the Kokish Relay sacrifices the natural 2 rebid by the strong 2 bidder, he must rebid 3H over 2D to show a natural heart suit.