A fixed deposit is a debt investment which means that the investor is lending money to an institution for a specific time period.
In a fixed deposit there is a pre determined time period for which the investment is made and this is known upfront like 91 days or 1 year or 3 years.
At the end of the time period the original amount invested is returned to the investor.
During the period of the investment there is an interest paid to the investor which is also known at the time of the investment.
This interest rate is locked in and thus the investor gets this return even if things change in the economy.