Basically it means when asked to open a 'subSilver' template file in the installation instructions you'd substitute 'subSilver' with the name of the style your actually using.
would be if your using the Aphrodite style;
THEN you'll be asking why you cannot find the code your asked to find in the 'FIND', well simply put;
The install instructions for EVERY mod support ONLY the default style supplied with phpBB which is 'subSilver'. After all how on earth is a mod author to know what style YOU are using. But then WHY does a style author have to conform to the 'subSilver' standard?
Oh! and Morpheus, MorpheusX, Aphrodite are a few of the non 'subSilver' based styles that are available for phpBB 2
Edit: And you have read this: viewtopic.php?t=2085