This page of the guide to Disco Elysium contains a detailed walkthrough description of the Fridge victim's body task.
The task Fridge victim's body in Disco Elysium appears if during the field autopsy (Perform field autopsy) you haven't been able to find all traces.
The moment you tell Kim that you must've missed something and you need to re-examine the body, he will answer that, in this case, you'll have to find a way to cool the corpse. Otherwise, the progressive decomposition process will prevent you from finding any more traces.
You can find a large Ice Bear Fridge find in the Doomed Commercial Area. After examining the fridge, you'll be able to tell Kim that the body can be moved there.
If you want to know how to reach the Ice Bear Fridge, look at Where is the fridge?.
It will take a moment before Harry and Kim manage to transport the body to the fridge, which will move the game time a little bit forward. You were able to stop the decomposition process, so you gained more time to examine more traces. Once the autopsy is complete, you will be able to send the body back and complete this stage of the investigation.