Yes, the abdominals are just like any other muscle. They need time to heal, and to become bigger, and stronger, they need to be worked with resistance.
Technically, resistance is anytime that your muscle is fighting to contract against some other force (including body weight and isometrics). However, I believe that he meant add some weight.
E.g. When you do crunches, do them with a 25 pound plate resting on your chest.