Here's the long version:
These are all the relevant posts from my journal.
For you specifically (this assumes the Strava based paid content acts in the same way as Elevate (free Google Chrome plug-in), which I believe they should since they're both based on Andrew Coggan's work).
It is important to note from the outset, that since you are not using a power meter (like Stryd), then the values calculated for this graph come from HR data. If your HR zones are setup incorrectly in the settings, then the data this graph generates is erroneous. Secondly, you're using Training Impulse which has a different scale than HRSS or power zones. TRIMP generates a much higher value on the graph than does HRSS. So keep that in mind if comparing your graph to others.
This graph is a measure of training load. How much you've done in the last 7 days and how much you've done in the last 42 days and how the two are related? Simply put, if you do WAY more in the last 7 days than you've been doing in the last 42 days you're likely to lead to overtraining.
For example - In Days 42 through 7, I've run 0 miles. In the last 7 days, I've run 100000 miles. Running that many miles in the last 7 days is inappropriate given what I've done the last 42 days. This graph would show that.
So,
Fitness = Training load for the last 42 days
Fatigue = Training load for the last 7 days
Form = Difference between Fitness and Fatigue
Training Impulse = The amount of training from a single day or single workout.
So the ideal "form" is debatable. This system was originally written for cycling. And then it has since been shoehorned into running. So whether the "form" zones are appropriate for running is debatable. But per the cycling zones, "optimal" training zone is anything between -10 to -30. You have a lower (more negative) than -30, and you're likely doing too much as it relates to the past. You're doing more (more closer to positive or positive) than -10, and you're likely in a recovery week or taper. The more time you spend in "optimal" in theory the better.
What does this graph not say?
-It does NOT tell you how fit you are. A "Fitness" of 67 is only how much training you've been doing lately. It doesn't mean you can run a 25 min 5k or a 17 min 5k.
-It does NOT tell you whether the training you are actually doing to make up the individual TSS (total stress score or daily score) is appropriate. I could do lots and lots of R (or mile) pacing, but on marathon day, that's not going to be terribly helpful. So specificity of training is lost in this graph. It ONLY tells you how much training you've been doing.
So now, more specifically to the graph:
-Your Fitness is 67.
-Your Fatigue is 86.
-Your Form is -19.
-Your run today was a Training Impulse of 46.
Per this data, the training you've done today has maintained you in an "optimal" training zone as it relates to the last 42 days and last 7 days. Presumably you are in a period of building, but not a rest/recovery week or taper week. The run today was 46 and as it relates to both Fitness and Fatigue it was far lower. Which means the intent of the day's run was likely an easy/recovery type day, but maintained a level of cumulative fatigue.