If you want to make some template customization without building a brand new
template, you should use this directory to store yur customized version of the
official template files rather than editing them on place.
Doing this, they won't be lost during the next upgrade.

For example, if you want your own header file just copy 
"template/yoga/header.tpl" to this directory with whatever, but significant,
filename you want.
Then edit "include/page_header.php" and replace the line :
  $template->set_filenames(array('header'=>'header.tpl'));
by
  $template->set_filenames(array('header'=>'../../template-extension/yoga/local/header.tpl'));
Or whatever new filename you gave instead of header.tpl

In future version of PWG, you probably won't have to edit the php code, but
a parameter table instead.

Feel free to ask in the forum for further assistance :
http://piwigo.org/forum