This summary is not the rule. The rule text controls.
Every witness who takes the stand must promise to tell the truth before they can testify. This promise can be an oath or an affirmation, and either one works the same way legally.
The point of this promise is to make the witness understand how important it is to be honest. The words used should be serious enough to remind the witness that lying under oath is a big deal. Courts can word this promise in different ways, as long as it gets the witness to genuinely commit to telling the truth.
This rule exists to protect the fairness of trials. When someone testifies without truly grasping the seriousness of their promise, it puts the truthfulness of their testimony in doubt. Requiring this oath or affirmation helps make sure witnesses take their testimony seriously and understand they could face consequences for lying.