The encryption key is used to encrypt and decrypt your data. Your login via email/password on the other hand just helps us to know who you are and to verify that it's really you. After you login successfully, we send your encrypted data to you. After you receive your data from us, you can decrypt the data with your key. So your key never leaves your computer and is never conveyed to us. You are the only person in the world how knows the key. This means, if you forget your key, you can't access your data and noone can help you.
Your encrypted data can't be decrypted without your key. Even the NSA can't do it ;) The only chance for an attacker is to guess your key. That is why you should use an encryption key which is not easily guessable.