The change isn't exactly monumental: Google Search has always returned results in a grid, so the basic layout is functionally identical to the current one. The biggest change is a visual one: the background in the test is a light gray, and individual search results are boxed in white with a small drop shadow to imply elevation.
Another major change is to the informative modules that Google shows for certain terms. Typing "Run The Jewels," for example, would bring up a list of search results accompanied by a card displaying information on the musicians and upcoming events on the right. In the new layout, this card is shown inline, before the regular search results. Given this is just a limited test, there's every chance this behavior will change if Google makes the Material Design layout the default.
Small changes elsewhere include new iconography for the magnifying glass in the search box, and switching out the settings cog for the three vertically aligned dots. Both the changes will be familiar to Android users. The google.com homepage has also been modified in line with the new aesthetic.
The test seems to have begun fairly recently, and on a small scale: There have been a few mentions of the new layout on Twitter and Reddit over the past week, and it's only showing up in one Engadget editor's search results. The company frequently trials new features and layouts on a small group of its users, seemingly at random. Many of the changes never make their way out of the testing phase. We've reached out to Google to ascertain its plans for the new design.