I’ll take a shot at how I use them, although I really think @kurt.braeckel covered most of it above.
I think of them as Fitness/Fatigue/Balance - so my apologies if I switch terms somewhere along the way.
Over the long term you want to see increases in your fitness. This is your long term stress, and as you train longer (think over the years) your body will become capable of handling higher levels of TSS consistently. So the overall trend for your fitness should be upwards, but gradually. In particular you will see increases in the fitness number as you progress through a base and build program (if you look at those plans the TSS increases gradually week over week. Whereas with a speciality plan the TSS numbers tend to be lower - so the burden on your body is less while providing more specific training.
Basically - you want a negative balance most of the time, and only want to go positive when you are in a taper for an event.
So, how do you use this? Well, pick a structured training plan and volume level that you think will work for you and have at it. If you can handle the stress of your indoor and outdoor rides and are still hitting your interval targets then all is good. If you start having trouble hitting your intervals and your FTP is correct then you are likely overtrained - take a look at your balance, note it down, and try to avoid going that deep again.
For reference, I am currently sitting at a fitness of 96, fatigue of 124, and thus a balance of -28. At this balance level I’m fine for base rides and that’s about it - I couldn’t do any VO2 work and would struggle with threshold work. But that’s fine, because I’m in Sweet Spot Base High Volume II so I know I can afford to go that negative to really build that pyramid base. Once I transition to a build plan I will be much more cautious with my fatigue number to ensure I am able to hit my interval targets.
For someone new to structured work I’d honestly recommend not watching any of these numbers that closely. Your body will answer most of these questions for you and the primary benefit of these is tracking longer term trends and tapering for specific events.