The idea is the following: I download an app, which uses CPU and GPU of my PC all the time to compute jobs of other users. In exchange, I can submit my job to the app. The system is very much like a torrent tracker with rating system, i.e. a user cannot use more resources of the system, than he gave to it.
The practical use: I can create a complex 3D scene in Houdini in 80 hours. The app uses 10% of my resources. Then, I can render the scene, which would normally require 8 hours in no time.
I don't have to pay with money, like in render farms, only with system resources