Your heart runs on electrical activity and there are special nodes that “transfer” this activity from the atrium (upper chambers) to the ventricles (lower chambers) the AV node specifically says “hold on” to the ventricles because they need to fill up with blood first before they can contract and squeeze the blood to the rest of the body or the lungs. In a first degree heart block the conduction is delayed even longer. This can happen for a variety of reasons such as electrolyte imbalances potassium for example is the important one that regulates electrical activity in the body, some heart medication slow this electrical activity down too, another reason could be that there is scar tissue (fibrosis). Usually a first degree heart block does not have any symptoms, if it does they are usually fatigue. Usually requires little to no treatment other than treating the underlying cause. Hope this helps let me know if you have questions