f-act/sec - This is the actual (real time) frame rate being processed in X-Plane. This number is the direct result of two other outputs, shown below
f-sim/sec - This is the frame rate the simulator is processing for the purpose of maintaining stability to the flight model. If you haven't adjusted your flight model, in the settings, disregard this for now.
frame time - This is the time required to render one frame, in seconds, based on the current position of your aircraft in the sim, taking into account of weather, scenery objects and mesh.
cpu time - A rate (in seconds) at which the cpu is processing one frame in the sim, and outputting to the frame rate time.
gpu time - A rate (in seconds) at which the gpu is processing one frame in the sim, and outputting to the frame rate time.