That’s the software built by server-side scripts, languages that build your site behind the scenes. To provide a seamless experience for the user that’s as close to a desktop application as possible.
There are many server-side languages working toward that end goal.
For example, if a user is updating a profile on a networking site, the server-side scripts will gather the information the user enters, the application will process it on the server, then interact with the database to update that information there.
Server-side scripts are used by back-end web developers to build the back-end software of a website—the mechanics we don’t see, but that make a site’s usability and functionality possible.
Database caching improves scalability by distributing query workload from backend to multiple cheap front-end systems.