The heat riser is a thermostatically operated valve in the exhaust manifold. It closes when the engine is cold, to direct hot exhaust gases to the intake manifold, in order to preheat the incoming fuel/air mixture. If it sticks closed, the result will be a rough idle after the engine warms up. If it sticks open, there will be frequent stalling during warmup, especially in cold and damp weather.
On the V6 and V8, the valve is between the exhaust manifold and the exhaust pipe. On the 6-232 and 6-258, it is an integral part of the exhaust manifold. The heat riser counterweight should move freely. If it sticks, apply Jeep Heat Valve Lubricant or something similar (engine cool) to the ends of the shaft. Sometimes rapping the end of the shaft sharply with a hammer (engine hot) will break it loose. If this fails, parts must be removed for repair or replacement.