Google recently launched Responsive Search Ads, which is one of their largest changes to search campaigns since expanded text ads were released in 2016.
Responsive Search Ads, currently still in beta, allow advertisers to add many different headlines and descriptions, which Google will test using its machine learning.
Think of this as multi-variate testing for your search campaigns, with less control over the combinations that you're actually testing.
Below is a screenshot of what the interface looks like when adding them to an ad group. As you can see, there's room to add many different headlines and descriptions:
What you need to know
These headline and descriptions can be shown in any order, so make sure they make sense on their own.
If there is specific ad copy which you'd like included in every ad variant, ensure that it is "pinned" by including it in either headline position 1, headline position 2, or description position 1.
You can explicitly pin each headline or description in the interface:
Finally, this is still in beta, so make sure you're tracking results closely. I'd definitely recommend testing these against your currently running ads to see how they do.
Why you should begin testing them ASAP
I've seen this scenario play out enough times to say that you're likely to get a bump in campaign performance by testing out this new feature.
It's usually the early adopters of these feature releases that experience the most benefit, simply because other advertisers in the space can't (or won't) implement this new ad type.
As mentioned earlier, Responsive Search Ads are still in beta. If you're a Google Partner (or your agency is), you should be able to request early access.
Owning more pixel real estate on mobile
While Responsive Search Ads are still in beta, Google Ads appears to have released a third headline and second description to all accounts (at least, this is what we're seeing with our clients).
This does give advertisers a chance to own more real estate on mobile search engine results pages (SERPs), as ads featuring three headlines and two descriptions will simply be larger.
Think about how you can take advantage of this if you're already maximizing ad extensions within your campaigns and getting strong top of page rates.

Shawn is a perpetual student of the game. Whether it's testing new campaign types or hunting down higher conversion rates for his clients, he's constantly on the prowl for an edge.
Want to work with him or talk shop? Fire him a message here.