Improving experience and expanding visual search features, Google Images search evolves to include captions for each image.
Initially for mobile browsers and Google app on iOS and Android, the caption information that is displayed with the image, is pulled from the title of the page that features the image. This feature should give images more context when shown on Google Images search results.
The captions appear above the domain address in bold text. This extra context makes it easier to for users to search for the right image.
Users may also visit the web page in order to see more images and learn more. It's like Google in combining regular search experience with Image results.
Initially, Google uses titles. But when asked whether those titles can be rewritten by Google, a spokesperson said: "We use web titles right now, but we’re continually experimenting with ways to improve the experience. Currently we use the web page’s title and nothing more.”
So here, they are based on websites' HTML title tag and is not used in any special way for ranking the image.
Google in showing captions displaying images, is how it tries to expand visual search feature.
This feature follows the removal of the "view image" and "search by image", which was a disappointment to many people.