Can someone please explain like I am 5, what is an NVR and what all can it accomplish?

I have a NVR and two cameras. I believe that the NVR acts as a computer/central server of sorts – am I on to something here? I would like to install about 15 total cameras (1 acre property) and phase out my current ADT system and replace it with a better DIY build. I have no idea if this is possible with my current NVR.

The security company that set up my equipment set me up with an app called DMSS. I HATE this app and it’s functionality. Despite the issues with ADT, their app is actually quite good and I can find movement events quite easily.

We have an old school ADT system for our actual alarm with about 8 cameras. However, it takes huge amounts of buffering time to actually get a live-view on any of the cameras, so this is very frustrating.

If you were in my shoes and wanted to phase out ADT and build an ultimate state of the art system , what would you do? I am not concerned about cost.