It is tied on hand after the sankalpa 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. You invoke God and ask for His/Her blessings that only good deeds may be done with your hand. The string represents the bond between Atma (soul) and Paramatma (God). You tie / bind yourself into the service of God. It represents our servitude to God.