John had a dual-chambered epicardial pacemaker. "Dual chambered" means both atrium and ventricle. "Epicardial" means the outside of the heart. So, in John's case, one lead was stitched on the outside of the atrium and the other on the outside of the ventricle. These leads deliver an electrical impulse to the heart to normalize its beating. The small battery pack can be inserted in various places. John had his placed in his upper abdomen.
Read more about the specific pacemaker John has on this website.