We offer our clients to trade CFD's, not the futures contracts that are traded in exchanges.
Any exchanges has quotes on multiple contracts at the same time, while our clients can only trade one contract with a particular underlying instrument. When the liquidity of the contract in question goes down, we substitute it with another one. This is actually what is called expiration.
You can avoid expiration by ticking the Automatically Roll Over to a New Contract checkbox, which means your position will be automatically re-opened once the contract has expired.
However, in case you have not ticked this checkbox, your transaction will expire at the current market price.
To view current expiration dates and contract months, please refer to the Instrument Specifications.