How does Plex work (for dummies)

I talked about it a bit in another thread here:

NextPVR is a free DVR program you can use with basically any TV tuner. It has a basic front-end you can use to watch, record, and schedule recordings of TV, but you can also integrate it with Kodi and such.

I tried Plex Pass but for the reasons I’ve described here or in the other thread I’m sticking with what I have today.