If you press the Tab key while viewing a published course, you may notice that a yellow box or rectangle appears around an object on the slide. If you repeatedly press the Tab key, the yellow box cycles through the items on the slide, then moves through the player controls for the course.
This is an accessibility feature in Articulate Storyline that assists learners who are using a screen reader. By default, the yellow box will travel from the upper-left corner of the slide to the lower-right corner.
You can control which objects display the yellow box and which ones don't. Bear in mind that if you turn off this accessibility feature for a crucial item on the slide, screen readers won't be able to communicate the necessary information to your learners.
Use the Size and Position window to disable this accessibility feature for any object on the slide. (It's enabled for all objects by default.) You can also specify alt text for any accessible object. The following video screencast demonstrates how: