A more thorough explanation:

Definition: not correct or accurate; not faithful or true to a standard or belief.

Examples:

• The statement he made about the company's profits was untrue.
• She promised to keep the secret, but she was untrue to her word and told everyone.

The first example illustrates the first part of the definition, where something said is not correct or accurate. The statement made about the company's profits was not true and therefore, it was untrue. The second example illustrates the second part of the definition, where a person is not faithful or true to a standard or belief. She promised to keep the secret, but she did not keep her promise and was untrue to her word.