It's a new block that terraform with version over 0.13 requires.
terraform {
backend "azurerm" {
resource_group_name = "tstate"
storage_account_name = "tstate20081"
container_name = "tstate"
key = "terraform.tfstate"
}
required_providers {
azurerm = {
source = "hashicorp/azurerm"
version = "~>2.0"
}
}
}