Before every puja or yagna, the priest ties a sacred thread on the hand of the person who is getting the puja done. This thread called, moli, is tied and the person is asked to take a sankalp which is like a vow, a purpose for which a particular puja or yagna is being done. It is tied on hand after the sankalp mantra-indicating a devotee, s resolution to worship , do puja etc. Hence the moli is a gross reminder and symbol of this conviction. As the string is tied around the hand, you pray that your life will similarly revolve around god. Once tied, one is not supposed to untie this thread for a particular period depending upon the type of ritual. Some people tie them up till next sankalp is taken. However, it usually continues for a week after the puja. Many follow this for two to four weeks. Raksha bandhan is one example where rakhi is tied by the sister on the brother, s wrist, who takes sankalp for the safety of the sister.