Load and daily TRIMP (training impulse) values in the graph are calculated here as:
minutes_in_zone_5 * zone_5_load_factor +
minutes_in_zone_4 * zone_4_load_factor +
... etc for all zones.
In that graph you can also see long (~6wk) and short-term (~1.5wk) exponential moving averages of trimp. So you can think of fatigue as your current load--that which is recent enough to make you tired--and fitness as the load you have been sustaining long enough to be mostly adapted to. Freshness is just the difference between those two. Hover or touch the graph to see the numerical values, though keep in mind that the units are arbitrary and relative. To see the graph for the current week, just click from the horizontal bar graph lower on your log.
To enable this on your own log, you have to set your own load factors via "Settings" > "Edit Intensities & HR zones" > third column. This is a bonus/upgrade feature so anyone who doesn't see this option can fix that by donating.
As far as what load factors to set, you may want to start with 0-5. I have personally set 0, 0.1, 0.2, 0.3, 0.6, 0.95 based on my own usage of intensity zones. You don't need to overthink this part initially, if you dive into the science below you can see that the original researchers got good results with just 3 categories and weights of 1,2,3.
Former physicists and curious types will enjoy some more background on the performance modeling here