A more thorough explanation:

A casual condition is an uncertain event that triggers or negates a duty to perform a promised action. It can be a stipulation or prerequisite in a contract, will, or other instrument, constituting the essence of the instrument. A casual condition can be a fact or event on the occurrence of which some legal right or duty comes into existence. It can be a condition that depends on chance and is not within the power of either party to an agreement. An example of a casual condition is when Jones promises to pay Smith $500 for repairing a car, Smith's failure to repair the car relieves Jones of the promise to pay. Another example is when an insurance company promises to pay £10,000 to an insured person if his house is destroyed by fire; the destruction of the house by fire is a condition of the insurer's promise to pay, but neither party promises to burn the house. These examples illustrate how a casual condition can trigger or negate a duty to perform a promised action. If the condition is not met, the promised action does not have to be performed.