It's for Browser extensions. Chrome extensions (and also Firefox and Safari). a fail called the "Manifest" that tells Chrome and the extension about the extension and contains all it's permission requests. Eg: "This extensions wants to modify all webpages", which is what ad blockers need to do.

Going forward extensions in the Chrome Web store will have to use Manifest v3. Which has less capabilities than previous version. Which will stop ad blockers working.

tl;dr. Ad blockers will not longer be possible in Chrome.