This page will cover each option in the Simulation Settings screen in-depth.
The cinematic mode checkbox, at this time, does nothing. In an upcoming release, it will switch three of the cameras to world-fixed cameras that track the robot.
This setting allows you to override the default gravity in the arena you've chosen. This is a fixed world-space vector. In the real world, on Earth's surface, it's roughly (0,-9.81,0). You can set the components of the vector individually, and there is technically no limit, although physics errors become more likely with higher values.
This setting allows you to override the default atmospheric density, and therefore aerodynamic drag, in the arena you've chosen. 1.0 represents Earth's atmosphere at sea level. Higher values increase the air density and drag.