Responsive Search Ads (RSAs) were a type of ad that contains a larger collection of assets in order to provide more flexibility to your marketing efforts.
While the total count & composition of assets are liable to change over time, the ads are currently created by combining 2 to 3 headlines and 1 to 2 descriptions.
Headlines are more prominent snippets of ad copy that can contain a max of 30 characters, while descriptions are beneath these in smaller font with a max of 90 characters. Each ad is allowed to contain up to 15 headlines and 4 descriptions.
These have replaced Expanded Text Ads (ETAs), which only allowed you to upload 3 headlines and 2 descriptions per ad.
Since June 30, 2022, you are no longer able to create or edit ETAs within Google Ads.