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:
The Session > Save Template shows a dialog asking for the name of the new template.
In the New Session dialog, a combo selector allows to choose the desired template.
An existing session can also be used 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.