A bundle branch block, (or BBB,) is a common problem with the heart. Indeed, it often isn't really much of a problem! The heart is composed of special muscle tissue that is sensitive to electricity. There are four chambers of the heart that the blood moves through. Since blood follows the same path through the heart, and it takes time for blood to move from one chamber to another, the electrical signal that triggers the heart to contract needs to follow a specific route so the right tissue can contract at the right time. Very basically, it works like this:
1) An electrical impulse is generated at the sino-atrial (SA) node, which is near the top of the heart. This causes the top chambers (the atria) to contract, pushing blood down into the bottom chambers (the ventricals.)
2) The impulse moves downward from the SA node to the atrioventricular (AV) node, which sends the impulse further down to prompt ventricular contraction.
3) From the AV node, the current runs into the Bundle of His (h-i-s-s), which splits into two halves - the right bundle and (wait for it...) the left bundle.
4) Both bundles carry the impulse to the terminus of the conduction pathway, the perkinjie (per-kin-gee) fibers. A bundle branch block occurs when there is impedence in the conduction of the impulse. That is, the electric impulse is slowed or stopped. Because heart tissue can make and conduct its own electrical impulse, a blockage is usually compensated for by literally rerouting the impulse through the heart tiisue itself. A slowing of the signal may make an EKG look different, but it's usually not that big of a deal.
Keep in mind that this is a very basic description of how the heart's electrical conduction works. More can be found on the web pretty easily.