When Internet companies questioned the effectiveness of video pre-roll, mid-roll, and post-roll ads, they came up with a new advertising concept: the ad overlay. An "ad overlay" is when certain videos contain a small, semi-transparent overlay across the bottom of the screen, similar to what you often see during TV shows. The ads show up 15 seconds into the videos and only last for 10 seconds.
Ad overlays can be a scroll, ticker, or watermark image. Viewers can click to close the ads, or they can click on them, at which point they stop the video in the background and open up a "player within the player" (known as in-player messaging) that can include more advertising content. Viewing video ads in this manner are called "in stream" because you are viewing it while it is streaming, as opposed to viewing an ad that appears over the browser. After the viewer is done with the ad, they can close it, and the original video picks up where it left off.