It is said of a secondary action plan, a contingency plan envisaged in case the main fails.
It means alternative, second option, escape route. Situation that can provide us with a viable alternative, when we notice that our first response is not effective.
PLAN B allerative plan, in case the main plan fails in its possibility of implementing it or at some point of its execution.
Referred to as " Plan B-" the secondary plan to follow where the main plan fails or is aborted by an unforeseen event.