The Origin:

It is often interesting to find the reason or significance of a festival. RakshaBandhan shares a similar background. The most popular one is from the Mahabharata and it acknowledges the relationship between Lord Krishna and Draupadi. According to the epic, one morning, when Lord Krishna was flying a kite, he cut his finger on the abrasive string. Draupadi, who was nearby, saw him bleeding profusely and ran to him. She then tore a piece from her sari and tied it around his finger. Lord Krishna was so touched that in return he promised to protect her from all evil, forever. And he did protect her all along, especially during her ‘cheerharan’ by the Kauravas

There’s another interesting story involving Emperor Humayun and Rani Karnavati of Mewar.

When Mewar was attacked twice by Bahadur Shah Zafar in the mid 16th century, she sent a letter with a RAKHI to Humayun as a last resort, to help her. The emperor was so touched by the gesture that he immediately left the military campaign he was then involved in to protect her.

Significance:

The festival of Rakhi is celebrated with zeal and vigor throughout India. There are many rites and rituals associated with Raksha Bandhan and some go beyond the more popular practice of tying Rakhi around the brother’s wrist. In some parts of India, families draw figures and symbols on the walls of their homes and worship them with offerings of vermilion and kheer. Palm imprints are also used to decorate the entrance of a household where Rakhi’s are stuck.