When adjusting fan speeds, Fan Control asks the operating system to run the fan(s) at certain speed(s).
The 'Lower Temp Threshold' setting defines the lowest temperature at which Fan Control may start requesting increased fan speed(s) to help reduce temperatures.
The 'Upper Temp Threshold' setting defines the temperature by which Fan Control will be requesting the maximum fan speed(s).
For each fan in the system:
the 'Min. Fan Speed' setting defines the slowest speed Fan Control will request for each fan
'Fan Speeds: Max.' shows the maximum fan speed allowed for the fan, as reported by the Operating System
'Fan Speeds: Wanted' shows the fan speed that Fan Control has requested and is presented as the circle on the graph
'Fan Speeds: Current' shows the current fan speed (and its percentage of maximum RPM) as reported by the Operating System and is presented as the horizontal line on the graph
'Current Temperatures' shows the highest CPU-related and GPU-related temperatures from the collection of CPU/GPU temperature sensors that Fan Control monitors for that system. Fan Control determines (what it considers to be) the most applicable collection of CPU-related and GPU-related temperature sensors then, every 3 seconds, determines the highest temperature from both of those collections.
Fan Control uses the highest of the shown CPU/GPU-related temperatures, represented by the vertical line on the graph, to compute the requested the fan speed(s) according to the User's slider/preference settings. Fan-speed requests are designed to keep each fan's 'Wanted' fan speed within 25 RPM of the sloped line on the graph.