[barkingcat]

It's about "distribution" of the software - which means that if a WP site is modified, and then given out to people to serve on their own servers (like a product that is based on Wordpress that contains for example, a ready made/installed curated set of plugins + themes), then this entire product must be opensource with the GPL.

If you are a person just running a WP site, using some custom plugins, BUT are not "Distributing" it, then you are ok.

The loophole of course is deciding whether sending out the html/css/js output of the site over the web to a visitor of your site counts as "distribution" (which is current "Not" by usual readings / interpretations of the GPL)