A charity called The Giving Pledge was created by billionaires Warren Buffett, plus Bill and Melinda Gates back in 2010. The pledge is for rich people to promise to give away a good portion of their money to charitable causes during their lifetime or at least earmark a ton of cash for good upon their death.
A total of 204 people have signed the pledge since it started. Bezos, the richest person in the world, is not one of those 204 people despite immense public pressure.
MacKenzie announced she is signing the pledge only weeks after her and the Amazon CEO reached a divorce settlement. In the deal she walked away with around $36 billion which means $18 billion will be put towards helping the world.