What is meant by heart block?

To understand heart block, we need to now an idea about the electricity of the heart.

The conduction system consists of Sino atrial node, atrioventricular node, His bundle, bundle branches, and Purkinje fibers. The block means there is some fault in the electricity center (Pacemaker) or their distribution (the conduction systemin the heart).

Heart block is either

Inherited (congenital), or
Acquired due to ischemia or degeneration.

Any delay in conduction will affect the impulse formation and propagation and so reflect on the heart function in 3 ways -

Not noticeable,
Mild occasional effect,
Serious effect which is rarely, but it is usually treatable.

Unexpectedly the blocks are more serious near to the end of the electrical path and less serious effects the closer they are to the start (SA node) here the AV node take over and escape rhythm originate 40-60/bpm.