This summary is not the rule. The rule text controls.
This rule deals with what happens if a court decides that part of the law is illegal or unconstitutional. It says that if one piece of the law gets struck down, the rest of the law still stays in effect. Courts don't have to throw out the entire law just because one part of it has a problem.
Think of the law like a chain of separate links. If one link breaks, the other links still hold together and keep working. This rule makes sure that a problem with one section doesn't bring down the whole set of laws around it.
This matters because it gives courts a clear instruction on how to handle legal challenges. Instead of guessing whether striking down one part means everything falls apart, judges know they should keep the rest of the law working as long as it still makes sense without the part that was removed.