Electrical impulses follow a pathway from the right upper chamber and procede to the right and left ventricles and the timing should be that both the right and left ventricles contract at the same time. A conduction delay indicates the electricle impulse to the right ventricle did not activate a contration at the normal time.
In right bundle branch block, contraction of the right ventricle is delayed, and in left bundle branch block contraction of the left ventricle (or some parts of the left ventricle) is delayed.
Probably not serious unless there are other symptoms or problems. It can be just a variation on an EKG and may or may not be normasl. Requires further testing if there are symptoms, etc.