A rate responsive pacemaker is one that changes its pacing rate depending on the activity levels of the patient.
When we are at rest we don't need to have a particularly fast heart rate - indeed most people's heart rate falls to about 40 beats per minute when they are asleep. As we become more physically active the heart rate rises to pump more blood around the body and particulary to the muscles involved in the physical exertion as these require more oxygen and sugar. In many people needing a pacemaker this ability to increase their heart rate with exercise is impaired or lost.
A rate responsive pacemaker has a sensor in it that responds to movement in the patient by incrementally increasing the pacing rate to the heart. Depending on how fit the patient is this can be set for a fast rate rise (less fit patient), moderate rate rise (average) or slow rate rise (fit). Typically the pacing rate can go up to a maximum heart rate of about 120 beats per minute.
Both single and dual chamber pacemakers can be rate responsive.