If you plan to walk home if the weather is nice, but bring subway fare just in case, then taking the subway is your contingency plan. A contingency is an event you can't be sure will happen or not.
The noun contingency describes something that might or might not happen. We use it to describe an event or situation that is a possible outcome but one that's impossible to predict with certainty. A company might have a contingency plan for what to do if something goes wrong with their primary strategy, also known as "plan B." They might even have plans C, D, and E, to prepare for multiple contingencies.