The announcement of this rule change with examples are at the bottom of the article. To summarize:

In the current rules, if you attack with a creature and it gets multi-blocked, once your opponent locks in their blocks, you choose the order of blockers immediately. During the combat damage step, you must assign enough (big asterisk on “enough”) damage on the first creature before you can assign damage on the next.

Starting with Foundations, you don’t choose an order for the blockers. During the combat damage step, you will just distribute damage however you want.

This weakens multi-blocking as a defensive option.

We will one day speak of blocker order like we do damage on the stack.