This summary is not the rule. The rule text controls.
Here's a plain English summary of this rule.
This rule deals with lawsuits that involve a group of people who could get bigger over time, like family members who might inherit property. If everyone currently in that group is part of the lawsuit, and the case decides something about a specific piece of property, the court's decision covers people who join the group later too. For example, if a lawsuit involves a family's future heirs and settles a property dispute, a grandchild born after the case ends is still bound by that decision, even though they weren't born yet when the case happened.
This rule exists so that court decisions about property can actually stick. Without it, someone could keep reopening old property disputes every time a new family member was born, since technically that new person wasn't part of the original case. Instead, the law treats everyone who is currently part of the group as representing the interests of anyone who might join that group in the future.
What this means for you is that if you are part of a class like this, whether currently included or born later, a court decision about the property in question applies to you too. You don't get a separate chance to challenge it just because you weren't around when the case was originally filed.