This summary is not the rule. The rule text controls.
Before anyone testifies in court, they have to promise to tell the truth. This can be done through a traditional oath, like swearing on a Bible, or through an affirmation, which is a similar promise without any religious wording. Either option works as long as it makes the witness understand they have a real duty to be honest.
The point of this rule is to remind witnesses that lying under oath has serious consequences. Courts allow some flexibility in how the promise is worded, so someone whose beliefs don't allow swearing an oath can still affirm to tell the truth instead. What matters most is that the witness clearly commits to being truthful before they start answering questions.
