Contango means the price of a commodity for delivery at a future date is significantly higher than the current or "spot" price, which makes it profitable for companies to store the commodity and enter into contracts to sell futures contracts on the commodity, even after accounting for storage costs and opportunity cost of capital (i.e. money tied down on the inventory not earning a return).
The situation persists untill the futures prices come down or current prices go up or the cost of storage becomes very high.
This situation is also referred to as an upward sloping forward curve.
The opposing market condition to contango is backwardation.