1. Used alone in response to a question about your intention to do something to mean that you will definitely do it. English = 'Promise!', 'I promise!', 'I swear!' and so on.
2. Used alone in response to a question about the truthfulness of what you have said to mean that what you have said is completely true and that you have been honest. English = 'Honestly!', 'Honest to God!', 'Cross my heart and hope to die!'