BL has two main "threads" for computation - the simulation and the renderer. The simulation thread is what runs the game logic, physics, AI, etc. When it's done, it makes a "snapshot" of the entire world and then hands that off to the renderer. In original BL, the simulation was fixed at 30 fps. For single player in BL PC, we allow you to change that to run at 60 fps, which is what the Hi Fi sim option does.
The render thread takes snapshots produced by the simulation and then does the work to send them to the GPU for rendering. In original BL, the renderer was tied to the simulation, so it could only make new frames as often as it got them from the simulation, i.e. 30 fps. In BL PC, if your computer is fast enough, the renderer will generate new snapshots that "interpolate" between ones provided by the simulation, which allows you to render faster than the sim. This feature works regardless of if your sim is 30 or 60 fps.