Electricity flows from the top of the heart to the bottom. Once the electricity flows from the atrium it accesses a pathway of electrical fibers that transmit the impulse to the ventricles.
If the main branches of this network get blocked, we end up with what is called a Bundle Branch Block. On the left side there are both front (anterior) and back (posterior) sections. If these get blocked, something called a hemiblock is developed.
Image source: https://www.nottingham.ac.uk/nursing/practice/resources/cardiology/images/cardiac_conduction.gif