Conditional contracts require certain conditions (or actions) to be met by one or both of the parties. Otherwise the contract does not go ahead — it becomes void. Both parties must agree to the conditions at the time the contract is made.
Real estate contracts often have conditions, eg you agree to buy a property as long as you can arrange finance in a certain number of days. If you can’t get finance, the sale will be cancelled and any deposit already paid will be refunded in full.
Car buyers often agree to buy a car on condition that the car passes a mechanical check. If the car fails to pass, the buyer can choose to cancel the contract and get any deposit already paid back in full