Google Search on desktop has started using carousels for videos in the search results as opposed to the older style video boxes that were being used before. The prior video box, or perhaps current for some users still as the changes may not be completely rolled out to everyone yet, doesn't look much different from the new carousel style but the main difference is that users will now see an arrow to the right of the box which initially contains three videos that they can click to scroll through additional results. Once they start scrolling a new arrow will pop up on the left side of the box that can be clicked to go back the other direction.
This isn't a completely new design for Google as this sort of "scroll-through" carousel list is already on mobile devices, so those that primarily use Google on smartphones or tablets to search for stuff are likely already familiar with the design and its ease of use. Aside from being a handy change for the user though it's also a benefit for the content creators on YouTube as it allows more possibilities to get "eyes on" for their content. Instead of just a few different thumbnails users will be able to sit and click through video after video after video all without having to leave Google, and should they not find the video they're looking for within a minute or two of scrolling they can simply move back up to the omnibox and refine the search with some different terms.
This also helps Google, as it technically has the potential to keep users on the site longer, and perhaps use it more as they won't have to navigate away to look through videos. Google has reportedly started rolling these changes out as early as a couple of weeks ago, with the major changes to include this carousel look said to have been implemented in the last few days.