A more thorough explanation:

An oath is a public promise that someone will do something or fulfill a duty, usually with the commitment to do so truthfully. It can also be a way of pledging support to a cause or entity. Oaths are often made in the name of a deity, but not always.

• Witness oath: In the legal system, a person must take an oath or affirmation that they will testify truthfully. This is often done by raising a hand and promising to tell "the truth, the whole truth, and nothing but the truth." Breaking this oath knowingly is a crime called perjury.
• Oath of citizenship: Those who want to become naturalized citizens of a country must take an oath of allegiance or citizenship. In the United States, this requires promising to uphold and support the Constitution, renouncing any connection to a foreign entity, and swearing allegiance to and defending the United States.
• Professional oath: Some professions require those entering them to swear an oath. For example, healthcare professionals take the Hippocratic Oath, which promises to perform their duties honestly, safely, and with integrity.
• Oath of office: All individuals elected or appointed to a public office must take an oath of office. For example, the President of the United States takes an oath to "preserve, protect and defend the Constitution of the United States." Other offices, such as the U.S. House of Representatives, the Senate, and the Supreme Court, also require an oath of office.

These examples illustrate how oaths are used in different contexts to ensure that people fulfill their duties truthfully and with integrity. Breaking an oath can have serious consequences, such as perjury or removal from a profession.