It just... is.

I honestly can't speak too much about htaccess and that variable. Some things I haven't totally wrapped my head around. .htaccess and preg_replace are two things I need to just force my way through.

Ok, but in your case, what is it passing? Do you know? And wouldn't it work the same without it?

Like you, pre-replace and regex are areas which I'm not sure I will ever understand fully. htaccess gets a little less confusng the more I work with it.

You are 'passing' the stuff after the url. It may work perfectly fine without it - I've never tried it. I just found something that worked for me and I've kept it.