In the world of bomb disposal, the phrases to the left of the boom and to the right of the boom were a way of talking about the a timeline before or after the detonation of a bomb.
Bomb disposal crews usually work to the left of the boom; emergency response crews usually work to the right of the boom--that is, afterwards.
Today, however, the expression has come into more general use, meaning the time leading up to and immediately after any disruptive event. Communities and organizations often speak about wanting to work to the left of the boom; but more sophisticated analysts argue that in some situations you want to act to the left, while in others, you're better off to the right.
The explosive roots of the expression bespeak the general sense of a population that feels itself almost constantly at risk.