Valve seals are like little caps that sit on the valve stems, under the springs, to keep the oil from the valve cover area from getting into the engine. The procedure is that you pressurize your combustion chamber so that your valve doesn't fall in, and then you can remove the spring to put in a new one. Before you put on the new spring, take off the old valve seal that you will then see and put on a new one.
Here are some valve seals on valve stems. For this picture, there are no springs installed.
EDIT: Ah, didn't get in before Dan.