Gray prototype type look when data is not loaded [on hold]

I have noticed recently that many sites use a gray prototype type look when loading data on page.

• Exhibit 1- Google drive

• Exhibit 2- Facebook

• Exhibit 3- Medium

Can some one explain what library is used for this effect and how does it work in principle? Is there a study/A-B test/research which suggests that this approach help in any way in keeping user engaged while content is loaded?