Rule 603 Oath or Affirmation to Testify Truthfully

Before a witness can speak in court, they must make a promise to tell the truth. This promise is called an oath or affirmation.

The promise can be religious (an oath) or non-religious (an affirmation), but either way, the witness must take it seriously. The point is to make sure the witness understands they have a real duty to be honest while testifying.