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 it must be taken seriously. The point is to make the witness understand that they are required to be honest. Lying after making this promise can result in a perjury charge.