How to create this hover effect

Site URL: https://www.rdbuilders.org/communities/new-berlin-tx/century-oaks

I would love to use this "hover" effect on property maps on our website but don't know where to start. The mentioned URL is not my website but the hover effect site. Can anyone tell me what this is called and where I might start to recreate something like this? 

We need to be able to see what you currently have going to be able to advise you on what might be done.

Please post the URL for a page on your site where we can see your issue.

If your site is not public please set up a site-wide password, if you've not already done so.

Post the password here.

Adding a site-wide password does not allow anyone to alter your site. It only allows those with the password to see your site. Please read the documentation at the link provided to understand how it works.

Please read the documentation at the link provided on how to share a link to your site to understand how it works. A link to the backend of the your site won’t work for us, i.e. a url that contains /config/.

We can then take a look at your issue.