The bank issues the certificates. They offer better rates than an online savings account. It’s a time-deposit account. You can invest for a specific period, like 3-4 months or 1-2 years. You get interested per month and principal + interest after CD’s period. Remember, you can face a fine if you remove cash before the period ends. People usually invest in CDs for a shorter period and reinvest them when the rates are higher.
These accounts are also FIDC insured, so the risk is shallow. Though, Inflation + Taxes can lose the purchasing power of the money. It’s a good fit for you if you don’t need immediate access to cash. You can buy CDs in banks and credit unions like Capital One and Barclays, among others.