I think they're referring to firebase which sells itself as a "realtime database" or rephrased, a database designed with "realtime" (automatically updating) websites/apps in mind.

The use of the word "realtime" for web tends to trigger a lot of systems developers who use the term to indicate that you can predict the actual real world time that something will take to compute, typically used in automotive and robotic settings. That being said, I didn't invent the word and words have multiple meanings and contexts. In this context it simply means push-delivered data that is pushed when updated. /disclaimer

It's a term that Firebase uses. From [their documentation](https://firebase.google.com/docs/database): "The Firebase Realtime Database is a cloud-hosted database. Data is stored as JSON and synchronized in realtime to every connected client."

I believe "realtime" in this case pertains to the synchronization of data amongst clients through websockets. This is how I've seen the term "realtime database" most commonly used.

People mention firebase but another one was (is? haven't followed it in recent years) RethinkDB.