what does this average frame time mean?

Hello,

The "Average Frame Time" is an average that is calculated from the frame durations in that area of the timeline. A frame duration is the amount of time a "surface" was on the display

Is this truly the frame time ?

Not necessarily, but it is derived from actual frame durations. If you had one very long frame duration, it could greatly skew the average at that point of the timeline. "Average Frame Duration" is calculated over smaller periods of time, it does not represent the average for the entire duration that your app was running.

Let's consider your example, this could be averaging 2 frames, the first frame's duration could be 16.6 ms (a typical 60 fps frame duration), and the second frame's duration could be 583.7 ms (a very long frame duration). These durations would then average to 300.15 ms, representing the "Average Frame Duration" across those two frames.