Custom templates allow you to write your own tag and variable definitions so that others within your organization can use them alongside the built-in tag and variable templates. The permission-centric, sandboxed nature of custom templates makes it possible to write custom tags and variables in a safer, more efficient manner than when using custom HTML tags and custom JavaScript variables.
Custom tag and variable templates are defined in the Templates section of Google Tag Manager. The main Templates screen will list any tag or variable templates that have already been defined in your container. You can also create new templates from this screen.
To learn more about custom templates, see the Tag Manager developer site.