Despite how poor they are, when Koly gets married her mom gives her some special silver earrings as part of her dowry:
The next morning Maa brought out the silver earrings she had worn as a bride. They were solid silver, and so heavy that when I tried them on, I was afraid my ears would stretch to the size of an elephant's. The bridegroom's family was satisfied. (1.18)
Since her family doesn't have much money, real silver earrings are a big deal. Not only are they expensive, and unusually so for the family, they mean a lot to her mom since they're the earrings she wore on her wedding day. To Koly, the earrings represent her family and heritage—they're a symbol of where she comes from as she heads out into the world. As readers, Koly's mom's willingness to part with these precious earrings also makes it clear that she loves her daughter, even if she's marrying her off for money.
Later, the silver earrings are Koly's ticket to get the heck out of Dodge when things go south with the Mehtas. Or makes that Mrs. Mehta, since by this point she's the last Mehta standing (Chandra's married and living elsewhere). Koly realizes pretty quickly that she can't trust Mrs. Mehta, so she hides the earrings and plans on using them to fund her way to freedom. Koly only gives the earrings up when she is can get something more valuable in return—kind of like her mother does when she includes them in Koly's dowry.