specifically a company insider, which means a person who owns 10% or more of voting stock or an officer or director. he technically can just sell shares, but the issue is that, being an insider, he likely has access to MNPI, knowledge of which can impact trading decisions and open up prosecution for insider trading. 10b5-1 provides a safe harbor to alleged insider trading accusations

Also when the announcement is made he HAS to sell, he can't back out last minute either.

they don't have to announce publicly - some do, but many others do not. and they technically are supposed to go forward with selling the stock but they absolutely can cancel. cancelling 10b5-1 plans has been a proposed loophole for a while.

the plans are to prevent insider trading. in the eyes of the SEC, insider trading occurs when an action has occurred - selling or buying stock. the act of not taking action according to SEC regulations does not constitute a trade.

might have changed recently though, so feel free to verify