what is 15 G?

Interst on FD is accrued every quarterly. If your estimated total interest on FD amount gets exceeded Rs. 10,000/- than TDS at 10% with PAN or 20% without PAN gets deducted and only the remaining amount gets accrued. By submitting the 15G (if applicable) you can save the TDS deduction and hence full amount of interest is given to you.

For details read: https://profit.ndtv.com/news/your-money/article-how-to-avoid-tds-on-bank-fixed-deposits-1456665