Din's Charm can be received after Link finds Din a house in Hyrule Town. After doing so, Din will offer him one of her charms; she will give him one if Link has an empty Bottle. While Din will give her charms for free, Link can only carry one at a time.
Din's Charm boosts Link's attack for a limited amount of time. Her Charm will turn Link orange and his eyes red if he uses it in the Mama's Cafe, but normally Link's tunic will just turn red.