Don't worry, you're not the only one that asks this question.
The FR display is showing the percentage of the range between your MIN and MAX setting that the control is calling for. This determination is based on the setpoint, your current temperature, and the history of what the control has done to meet that setpoint.
The MIN and MAX define the number of seconds the stoker motor is on within each 100 second window of time. So a MIN of 6 means that at idle it will run the stoker motor for 6 seconds for every 94 seconds it is off. MAX of 40 means that running the stoker motor for 40 seconds for every 60 off will produce a full burn on the grate.
So the FR will be a number between 0 and 99, with 0 representing the use of the MIN setting and 99 representing the use of MAX. Values in between 0 and 99 calculate a percentage between MIN and MAX.
If you want figure out how much on and off time a given FR represents you can use this equation: On time = (MAX - MIN) x FR% + MIN, Off Time = 100 - On Time. So if MIN is 6 and MAX is 40, a FR of 73 means: (40 - 6) x 73% + 6 = 30.8 seconds on time. We drop the fractions so really this means 30 seconds of on time and 70 seconds of off time.