In the above sequence, the first response, 1s is forcing, since it is not a "limit" bid. Responder may have a "1 over 1" bid of 6-9 points, but may also have a 10-12 hand with which s/he would have bid two of a minor if the best suit were a minor instead of spades. 1NT is non-forcing because it strictly limits the responder to 6-9 points.
The responder's second bid, 2d, is forcing, if you are playing "fourth suit forcing" as most players do, nowadays. Responder is saying, I need a third chance to fully describe my hand.
The openers rebid (third bid), 2c, is not forcing. Opener has bid two suits, failed to raise the responder's suit, spades, and doesn't like NT, even with responder's spade bid. The opener implies a hand that is limited in some way (probably weak diamonds). Responder can pass if the third suit (clubs) is reasonably suitable for play (xxxx, or Kxx or Axx in responder's hand). There is a potential misfit unless the responder can cover diamonds, and an alternative contract might be worse. With two little hearts, and weak clubs, responder should go back to two hearts.