Explanation
Azure Functions deployment slots allow your function app to run different instances called "slots". Slots are different environments exposed via a publicly available endpoint. One app instance is always mapped to the production slot, and you can swap instances assigned to a slot on demand.
There are a number of advantages to using deployment slots. The following scenarios describe common uses for slots:
* Different environments for different purposes: Using different slots gives you the opportunity to differentiate app instances before swapping to production or a staging slot.
* Easy fallbacks: After a swap with production, the slot with a previously staged app now has the previous production app. If the changes swapped into the production slot aren't as you expect, you can immediately reverse the swap to get your "last known good instance" back.
* Prewarming
Reference:
https://docs.microsoft.com/en-us/azure/azure-functions/functions-deployment-slots