Kokish relay is the sequence 2♣-2♦-2♥-2♠. If this convention is on, a 2♥ rebid by opener after a strong 2♣ opening shows either ♥s or a big balanced hand (25+ HCPs). Responder must then relay to 2♠. If opener then bids 2NT, it shows a big balanced hand. Otherwise, it shows ♥s.
The traditional way of showing a big balanced hand (25+) is to open 3NT directly. However, as these hands are rare, it has been replaced by the sequence 2♣ - 2x - 3NT. Both sequence eat up a lot of bidding space so a responder with a weak hand is difficult to respond. This convention saves a level of the bidding space.