I don't think it's hard to understand... the poster of this thread seemed to have an exceptionally hard time understanding it though, and BG3 doesn't handle its tooltip explanations well.
A tooltip that shows up when you hover your AC should explain how that number is arrived at. If it 'just' says '16 because of barkskin' when you're under 16, then it's not giving information that may be relevant - such as how close to 16 you are without barkskin adjusting, and thus whether a particular buff would be worthwhile or not. That's why I suggested that ideally, the tooltip should show what your current best AC calculation method is, with enough information to show you what it would be before Barkskin bumps it up to 16. A player wondering whether they should cast Shield of Faith on an ally may be in a situation where casting the spell might raise their AC to 18 (if the target's Ac is 16 even without barkskin), or it might do nothing at all (if it is 14 or lower without Barkskin's adjustment) - and they should be able to tell based on information the game makes available to them which is the case with a quick hover check.
