Credit points are like...... points that you owe to someone in return until you get something that people want in return.
For example - You just trade a useless pokemon for whatever pokemon you want from me. Each of my pokemon cost 1 credit point. So I trade you one of my pokemon for a credit point, I save it by putting it on my trade thread, and wait until I see what pokemon you have that grabs my interest. Does that make sense?