Templates can be opened as a new document with pre-populated layers. Sketch comes with several useful sample templates, including the iOS App Icon and Material Design templates.
You’ll see a grid view of templates in the Documents window. You can bring this up at any time by choosing Window › Documents. You can also get a list of templates by choosing File › New From Template….
To save a document you’ve created as a template, choose File › Save as Template….
You can also manually save and organize Templates into folders, as well as give them custom preview images in the Finder. You’ll find them in:
How to Create Custom Template Previews
To give your templates a custom preview, add a PNG image with the same filename as your template to its location in the Finder. For pixel-perfect previews, images should be 188 × 188 px for Retina displays, or 94 × 94 px otherwise.