An order is a way of organizing things or doing them step by step. To reorder something means to change its original order.
When we talk about reordering, it usually means making some changes to how things are arranged or done. For example, if you have a lot of tasks to do for work and your boss tells you to prioritize the most important ones first, you'll need to reorder your list so that those tasks come before the others. If you're buying tickets online, but they were sold in the wrong order - with the most expensive ticket first - you might want to reorder them by price so it's clearer what you're getting.
(transitive) To place in a new order; to rearrange.
Example: The books were sorted by title, but I'm going to reorder them by author's surname.
