This error message is shown in DubBot when code is detected that would cause the "tab focus" to appear lost to a web visitor.
Web visitors who utilize a screen reader typically rely on using a keyboard to tab through a page versus scrolling through the page using a mouse. Other users may also prefer using a keyboard over a mouse for speed or simply because of limitations of access to a mouse.
Maintaining a visual tab focus is important so that web visitors utilizing a keyboard for browsing know where they are within the page's content. The tab focus will often allow users to click on links to navigate throughout a website.
Tab focus can appear lost if the focus goes into a menu that expands when it is collapsed. Tab focus can also appear lost if the focus goes to an element within a slideshow when the element is not currently the one displayed on the screen. Simply put, tab focus appears lost whenever a user is able to tab to something not displayed on the screen.
A simple way to test tab focus is to tab through a web page and verify that you are not ever taken to an element that is not displayed on the web page.