A more thorough explanation:

A condition precedent is a requirement that must be met before a right, claim, duty, or interest can arise. It is different from a condition subsequent, which is a requirement that must be met after a right, claim, duty, or interest has already arisen.

In a contract, a condition precedent might be that a certain event must occur before the parties are obligated to perform. For example, an insurance contract might require the insurer to pay to rebuild a customer's home if it is destroyed by fire during the policy period. The fire is a condition precedent because it must occur before the insurer is obligated to pay.

In property law, a condition precedent might be an event that must occur before a property interest can vest. For example, if parents write a grant that states they grant the property Blackacre "to our son, if he graduates from college by his 21st birthday," graduating from college before the son's 21st birthday is the condition precedent. If the son graduates from college before his 21st birthday, title to Blackacre transfers to him. If the son's 21st birthday comes and he has not yet graduated from college, the interest never vests, and the parents never lose ownership of Blackacre.

Courts prefer to interpret a clause in a contract as a promise rather than a condition precedent to avoid forfeiture. The Second Restatement of Contracts has dropped the term "condition precedent" and simply refers to it as "condition." Conditions precedent are most frequently seen in combination with contingent remainders. Accordingly, conditions precedent are usually subject to the rule against perpetuities, which states that they are invalid unless they must vest, if at all, within 21 years after a relevant life in being.