A more thorough explanation:

An executive session is a meeting where only certain people are allowed to attend. It is usually held in secret and can only be attended by members and invited nonmembers. The term originated in the United States Senate, where it was used to advise the President about executive business such as appointments and treaties.

For example, a company's board of directors may hold an executive session to discuss sensitive information that should not be shared with the public or other employees. Another example is a government body holding an executive session to discuss confidential matters such as national security or personnel issues.