A Template Song is a special song file used as the initial document when Renoise is launched or when creating a new song. By default, a new song is completely blank. However, if you would like new songs to be created with a specific set of tracks, DSP effects, instruments, MIDI mappings etc. then you can create such a base song and save it as a template by selecting "File -> Save As Template Song" from the menu options in the Upper Status Bar.
To delete the template, making new songs blank again, choose "File -> Delete Template Song". It’s also possible to bypass the Template Song and use a blank one instead, without having to delete the template, by selecting "File -> New Song (no Template)".
You may find it useful to watch a short video on this subject.