What sort of problem is an atrio-ventricular node conduction block?

AVB has 3 flavors. There is 1st, 2nd and 3rd degree av block. 1st degree is just slower conduction through av node than normal. In 2nd degree there is a progressive slowing of conduction until complete failure to get through and then the cycle repeats itself. It is usually not symptomatic. In 3rd degree there is no conduction getting through and the ventricles beat by an escape beat that is usually very slow.
AV block. The av node is specialized tissue in the heart that normally modifies/delays electrical impulses transmitted from the upper chambers (atria) to the lower chambers (ventricles). If the delay is too intense, conduction slows abnormally and "block" is said to be present. The severity can vary from insignificant /no symptoms to passing out, even death. A simple ekg can usually diagnose the severity.