2of2 accounts simply mean that for the access to the account to be granted, two signatures have to be provided. The first signature is only known to you while the other one is held by GreenAddress. No transaction can be signed by GreenAddress without your signature and neither can you make any transaction without the key held by GreenAddress. In 2of3 account, two out of the three signatures are required to authorize transactions. The third key, which is held by you comes as a backup in the event that GreenAddress service is unavailable and therefore cannot provide the other key.