Question
You have heard there is such a thing as customer provisioners (not providers) in Terraform. What is a custom-built provisioner in Terraform?
Answer
There are generic provisioners such as "file", "local-exec", and "remote-exec".
To learn about provisioners, see this: https://www.terraform.io/docs/language/resources/provisioners/
Custom provisioners exist; you can create your own provisioner beyond the generic three ("file", "local-exec", and "remote-exec"). To learn more about writing your own (e.g., a plugin or special word for a Terraform provisioner), you can read this GitHub.com posting.