The Center of Pressure (CP) is the point at which the aerodynamic forces on a rocket in flight are centered. CP should not be confused with CG, the Center of Gravity. CG is the center of the mass or weight of the rocket.
It's not hard to visualize the Center of Gravity as the point on which you could balance the rocket.
The Center of Pressure is a little harder to visualize. Imagine that you're trying to balance the rocket on the same point, but inside a wind tunnel with the wind blowing down on the rocket. The point where you can balance the rocket will be determined not by the weight of the parts of the rocket, but, instead, by how hard each part is pressed by the wind. The CP balance point will be in a different place than the CG balance point.
In the earth's atmosphere, a model rocket is affected by both air and gravity. The air pressure tries to rotate the rocket around the Center of Pressure while gravity tries to rotate it around the Center of Gravity. We take advantage of this to stabilize our rockets in flight.
For your rocket to be stable in flight, the CP must to be below (aft) of its CG. A typical rule of thumb is to have the CP "1 caliber" or more behind the CG (where "1 caliber" is equal to the diameter of the main body tube).
A rocket is almost always unstable if the CP is not located below the CG. An unstable rocket will often loop after leaving the launch pad. Even worse, an unstable rocket may become stable as the rocket's motor burns porpellant and becomes lighter. This can cause the rocket to suddenly shoot off in a random direction - a potentially dangerous situation.
A rocket with its CP below the CG is usually stable. The further behind the CG that you place the CP, the greater the stability of the rocket. However, an over-stable model has its own problems.
An over-stable model will be greatly influenced by wind and cross-breezes. An over-stable model will have a greater tendency to come off the launch pad at a sharp angle if there is a breeze at the launch site.