There are three fascicles in the heart. They can be thought of as the electrical system of the heart with each fascicle being a separate cable that transfers electical impulses to various parts of the heart. A left antarior block implies that one of those fasciles isn't working as well and the other two are filling in for it. This may be related to a prior heart attack, but more so due to an older age and degenerative disease in the conduction system. It does not portend a poor prognosis.
One of the nerves on your heart isn't working as it should do, the one feeding a signal into your left ventricle. They call it a block, but the signal is still activating your left ventricle, causing it to pump but there's a very slight delay. You was probably born with this defect and nothing can be done to correct it unless you have a pacemaker but there would have to be a concerned delay in the left ventricle for this. You are at greater risk of needing a pacemaker at some time with this condition and It's not really dangerous.