This summary is not the rule. The rule text controls.
If someone tries to use an out-of-court statement to prove something in court, that statement usually can't be used as evidence. This is called hearsay, and courts generally don't allow it because the person who originally made the statement isn't there to be questioned about it.
There are exceptions to this rule, though. Other evidence rules spell out specific situations where hearsay can still be allowed in court. So while hearsay is blocked by default, it's not automatically kept out in every case. If a specific exception applies, the statement may still come in as evidence.
The main idea behind this rule is fairness. Courts want to make sure that when someone testifies about what another person said, the other side has a chance to question the original speaker directly. Without that chance to cross-examine, statements made outside of court are seen as less reliable, so they're only allowed in when the rules specifically say it's okay.
