A mining contract is an agreement in which a buyer pays a provider up front for a share of its mining power, measured in hashrate, for a set period, and receives the coins that share earns, minus fees. These contracts are the product sold in cloud mining, which is explained in full on that page, including its long record of fraud.
Whether a contract pays off depends on coin prices, mining difficulty and the provider's honesty; buyers can work out the expected return on investment before signing.