This summary is not the rule. The rule text controls.
Before a trial starts, once the jury has been picked, the judge asks everyone on the jury to make a promise. This includes the alternate jurors, not just the main jurors who will decide the case. The judge asks them to swear or affirm that they will fairly look at the case and reach a true verdict based on the law and the evidence presented.
This promise is called an oath or affirmation. Jurors can choose either option depending on their personal beliefs. Swearing usually has a religious tone, while affirming does not. Either way, the jurors are making a formal commitment to do their job honestly and fairly.
This step happens for a reason. It reminds jurors that their role is serious and that they need to base their decision only on what happens in the courtroom. It also holds them accountable, since they have publicly agreed to follow the rules of the trial before any evidence is presented.