This summary is not the rule. The rule text controls.
If you want to testify about something in court, you have to actually know about it firsthand. This means you saw it, heard it, or experienced it yourself. You can't testify about things you only heard secondhand or things you're just guessing about.
There has to be enough proof that you really do know what you're talking about. The good news is that your own testimony can serve as that proof. You can simply explain how you know what you know, like saying you were standing right there when something happened.
There's one special catch about hypnosis. If you only remember something because you were hypnotized, that memory doesn't count as personal knowledge. Courts don't trust memories that surface during or after hypnosis, so you can't use those to testify about what happened.
This rule works differently for expert witnesses. Experts are allowed to give opinions based on their training and experience, even without firsthand knowledge of the specific events in the case. That's covered under a separate rule about expert testimony.
