The steering is a value that goes from -1.0 (full left) to +1.0 (full right), being 0.0 the center.
Movement Rate is the rate of motion towards left or right, in value/second, applied to the steering when the Left or Right inputs are applied.
For example, if movement rate is 0.5 then pressing Right will take 2 seconds to the steering value to reach 1.0 (full right).
Auto Center Rate is the rate of motion towards the center (0.0), in value/second, applied to the steering when no steering inputs are applied.
For example, if auto center rate is 0.2 and the steering is at full right (1.0) then releasing the steering keys will take 5 seconds to the steering to return to the center (0.0). If auto center rate is 0 then the steering won't return to the center by itself.
Personally, I find the progressive steering mode useful for driving simulated trucks with the keyboard or joystick. Some old commercial truck simulators implemented it, and I had a lot of fun playing with them without having to set up a steering wheel device.