Fixed deposit is investment instruments offered by banks and non-banking financial companies, where you can deposit money for a higher rate of interest than savings accounts. You can deposit a lump sum of money in fixed deposit for a specific period, which varies for every financier.
Once the money is invested with a reliable financier, it starts earning an interest based on the duration of the deposit. Usually, the defining criteria for FD is that the money cannot be withdrawn before maturity, but you may withdraw them after paying a penalty.