Session templates are a way to store the setup of a session for future use. They do not store any audio data but can store:
Choose Session > Save Template. A dialog will ask you for the name of the new template.
In the New Session dialog, choose the desired template from the combo selector.
Note that you can also use an existing session as a template, without saving it as one. This is available as an option in the New Session dialog. Doing this will not alter the existing session at all, but will use its track, bus and plugin configuration just like a template.
See also Adding Tracks and Busses for information on templates for individual tracks or busses.