A more thorough explanation:

Definition: An heir apparent is someone who is guaranteed to inherit property from an estate. This means that their right to inherit cannot be taken away, unless they die before the person they were supposed to inherit from or if they are legally excluded by a legitimate will.

Prince Charles is the heir apparent to the British throne. This means that he is guaranteed to become king after Queen Elizabeth II passes away.

John is the only son of his parents and they have no other children. He is the heir apparent to their estate and will inherit everything they own when they pass away.

These examples illustrate how an heir apparent is someone who is certain to inherit property or a title. In the case of Prince Charles, he is guaranteed to become king because he is the eldest son of the current monarch. In the case of John, he is the only child of his parents and therefore the only one who can inherit their estate.