This issue occurs when content within an iframe is inserted but does not have either a title or aria-label.
The Siteimprove page report for this issue will look similar to the one in the image below.
In this example we can see a youtube video was added into a content block. To resolve the flag, we need can add either a title or an aria-label with a description of the content. Semantic html is usually preferable to using aria attributes where possible, so a title is usually a better solution. Start by clicking on the edit button of the content block widget and then click to the html view. Here you can see we added a title of "interactive map showing location of Student Wellness Center" to this video. The title or aria-label should provide a good explanation to a screen reader of what the content in the iframe is.