The primer is the small, round, metal cup that is in the center of the case head. It contains a small explosive charge that is sensitive to impact and is used to ignite the powder. When you resized the case you also pushed out the spent primer. The next step is to replace it with a new primer. There are two kinds of rifle primers, standard and magnum, and two sizes in each, large and small. (The same applies for pistol primers, which are slightly different than rifle primers.) Your loading manual will explain which one you need for the load you have selected. This is important. The primer is “press fit” by force into the primer pocket. Most reloading presses have a priming feature to seat the new primer. However, a hand held tool is much easier to use and should be bought when possible. You should seat the primer fully so that the legs of the anvil are all contacting the bottom of the primer pocket. (The anvil is the triangle shaped, three-legged piece of metal you see in the bottom of the primer. Too much pressure will deform the primer, damage the priming compound and possibly cause erratic ignition.)