Baz it means never cross someone else going up or down the stairs in an opposite direction to you. Either wait at the top or bottom for the other person to complete. I've not made it up:-
"Superstitious folk believe that passing others on the stairs brings bad luck, citing biblical passages where angels passed one another in the opposite direction while traveling on a ladder [source: Webster]. This may be as good a reason as any to avoid passing on stairs, but if you don't consider yourself superstitious, consider this: Maybe passing on the stairs brings bad luck simply because you're attempting to pass someone in close quarters, and one of you could trip or fall as a result. Better just wait until the other person exits the staircase before making your move"