ARM templates define the infrastructure and configuration of your Azure solution. Azure is managed by an API, which is called the Resource Manager or ARM API. You can use this API to deploy infrastructure as code and configure your Azure environment. This API can be called from various tooling and resources; you can do it using the Azure portal, PowerShell, or the CLI, or by calling the API directly, or creating ARM templates.
You can create an ARM template in JSON format and use this to repeatedly deploy your solution across your Azure environment in a consistent state. The template is processed by Resource Manager like any other request, and it will parse the template and convert the syntax into REST API operations for the appropriate resource providers. The REST API uses the resources section inside the template to call the resource-specific APIs. An example of a resource provider is Microsoft.Storage/storageAccounts.