Durable Functions allow us to develop reliable execution of our functions, which means that even if the VMs crashed or are restarted while the function is running, it automatically resumes back to its previous state automatically. It does so with the help of something called checkpointing and replaying, where the history of the execution is stored in the storage table.
You can learn more about this feature at https://docs.microsoft.com/en-us/azure/azure-functions/durable-functions-checkpointing-and-replay.