Currently, your survivor needs about 2500 kcals every day. (This is old info - it’s now based on your activity)
Hunger is different from your physical caloric needs, and is instead based on the volume of food in your stomach - and the time since you last ate
If you are hungry, but don’t really need to eat - your hunger display will show peckish
If you are hungry, but need to eat - your hunger display will show hungry
If you are hungry, and have a kcal deficit - your hunger display will show very hungry, then famished, and then with a severe deficit starving
Your kcal count is internally measured out of 55000 kcals (formerly 77000), and you can see how much in deficit/surplus you are by turning on debug mode.
Quoting the person who made it:
This is probably you misunderstanding the display - if it’s 60,000/77,000 you are in a severe kcal deficit. If it’s 137,000, then yes, you have found a bug, and please report it on GitHub with an attached save.
These descriptions are getting tweaked to be more helpful.
But you don’t need to know how the system works - what you need to do is eat when you’re hungry, and get enough kcals to not have a deficit.