I do appologise for keeping asking you questions, I just can't get my head round this

I have modified the line in functions.php which you told me, but I don't understand the showthread part.
Would I add something to this line in showthread.php?
"$pagenav = getpagenav($totalposts,"showthread.php?threadid=$t hreadid&perpage=$perpage".iif(isset($highlight), "&highlight=$highlight", ""));"