I can't find code {thumbnails}

[Sun]

I have a problem: sometimes i can't find code which i want to change (i can't understand what i need to do step by step for finding code).
Today i trying to find code which called with this expression: {thumbnails} (catigories.html) .
Please, tell me where i can find this code.
If you can explain me how i can understand where to find code if it called like {....} than i will be very happy.
Thank you.

[impss]

That template tag is built in your categories.php as
$thumbnails

and then is registered as a template tag here
$site_template->register_vars("thumbnails", $thumbnails);