A "Cap" is a warm, dry and stable layer in the atmosphere where the temperature begins to increase as you go further up in altitude. Often times especially in the plains there is a "Cap" in place. A "Cap" can prevent storm development at least in the short term most of the time but if a "Cap" remains strong little to no storms develop but most of the time it breaks eventually. When there is a "Cap" Air parcels rising into this layer become cooler than the surrounding air, which prevents their ability to rise further and produce thunderstorms.