You specify the websites that you want to block on your iOS device, which are then synchronized across your other iOS and macOS devices over the iCloud.
The rules are based on regular expressions, which allows for fine control over a blocked URL. For example you can block the landing page of YouTube and still watch individual videos on YouTube.
To help you stick to your rules about:blank has an option that requires you to entering a random string to delete existing rules.
Furthermore you can block websites on the fly through an iOS safari action.