This summary is not the rule. The rule text controls.
This rule covers how and when a criminal case can be dropped, either by the prosecutor or by the judge.
The first part deals with the government dropping charges. Prosecutors can decide to drop a case before it's finished. Unless they say otherwise, dropping the case this way doesn't stop them from filing the same charges again later. However, once a trial has actually started, prosecutors cannot drop the case without the defendant agreeing to it. This protects defendants from being pulled into a trial and then having the government back out in a way that could hurt them, like refiling charges later to gain some advantage.
The second part gives judges the power to dismiss a case on their own. If the prosecutor takes too long to file formal charges, or if the case drags on too long before going to trial, the judge can throw the case out. This rule exists to keep cases moving and to stop defendants from being stuck waiting indefinitely without knowing what will happen to them.