Templates are files created with reusable content which you can quickly add to another file or use as a starting point when creating a new file.
To create a new file based on a template, click + From Template in the file selection menu and select a template from your own folder or browse and select one from the Whimsical templates gallery:

Note: If the template you've chosen has a custom color theme added to it, it will be applied to the entire file.
If you want to add a template into an existing file, click Insert Template under the drop-down menu next to the file name, then choose a template:

You can also add templates to your board straight from the toolbar menu:

To create a new template, go to Templates and themes, click New template, and choose a file type:

You can also add a custom color theme to your templates. Learn more about themes and how to add them to templates here.
If you already have a file that you'd like to use as a template, you can simply copy or move it to your Templates & Themes folder.
You can easily save your favorite or most used files as templates for your team to reuse again. Simply click Save as template under the drop-down menu next to the file name:

Your template will be under the Templates & Themes section of your workspace and can also be found in the template browser when importing a template or creating a new file from a template.
Check out the Whimsical template gallery for some ideas and inspiration.