The app works it out on its own when each session ends, from what you logged. It does not ask how you feel: it looks at the numbers.
What raises it
- Falling below the range. The plan asked for 8 to 12 and one set came out at 6.
- Dropping the weight between sets. You started at 60 kg and finished at 50 kg.
- Moving a lot less than the last time you did that day.
- Taking sets almost to failure, with an RIR of 0 or 1.
Two of those in the same session make fatigue high. None of them plus some improvement, like a record or more reps than asked, makes it low. Anything else is normal.
The first week does not count
In the first week of a plan you are looking for the right weight. Lowering the load or coming up short there adds no fatigue.
What to do with it
One session with high fatigue changes nothing: it can be a bad night. What matters is it repeating week after week along with sets that miss the range. That is when a deload week makes sense, and then you build back up.