Reading order
Reading order is about the logical order something is read
A sighted user will skim across the content on a page or presentation, see the layout and that will help them read the content as it is visually displayed.
The problem for accessibility is that most screen reading tools will read out content in the order that it was created or coded. This will be the same order for someone only using the tab key on a keyboard if they can't use a mouse.
Take this example image of layout made in PowerPoint to introduce members of a team. It is created as three sections that each include an image, a person's name and a description of what that person does.
A visual reader would see and read
An image of Robert, Robert, he is the head of the company and is the boss.
An image of Ellen, Ellen, she organised all of the company events in the UK.
An image of Marta, Marta, she creates all of the marketing materials.
Hearing content in the order it was created
If the reading order hadn't been checked and adjusted to be made logical, a person tabbing through the content or hearing it through a screen reader would hear it in the order it was created.
Imagine this was in the order of all names first, then the descriptions and then the images added later, this is what they would hear.
Robert, Ellen, Marta, He is the head of the company and is the boss, she organised all of the company events in the UK, she creates all of the marketing materials. Image of Robert, image of Ellen, image of Marta.
Now imagine if you could only hear this content and not see it to match it up. It's not clear who does what.
Reading order in PowerPoint
PowerPoint has a reading order pane to help you easily re-order your content to make logical sense and match the order of how a visual user reads the page. This video describes how you can simply drag and drop the content into the right reading order.
Here is guidance for PowerPoint reading order from Microsoft