When the police "hold" a person, they keep them in jail.

If a person has been convicted of a crime and sent to prison, you don't use the word "hold". You use "hold" for a temporary stay in a jail. Usually a person is "held" by the police for a day or a few weeks:

We're going to hold him here overnight.

He's being held in the Vance County jail on suspicion of drug trafficking.

However, in some countries, the police can "hold" a suspect "without a trial" for a lot longer. This means that the police don't have to prove that you committed a crime. They have the power to hold people in jail for as long as they want.