Understandable means fair enough, a reaction anyone might have had in the same spot. Your friend snaps after four hours in an airport queue, and you say it is understandable. You are not saying she was right; you are saying you can see how she got there.
The other sense is the plain one built into the word: able to be understood. An understandable explanation is one you can follow, understandable English is English that gets through. Because 'clear' does that job better in most sentences, the excusing sense has taken over, and when a native speaker says understandable they nearly always mean it makes sense that you felt that way. The adverb understandably does the same excusing work: 'She was understandably furious.' The opposite is incomprehensible for the clarity sense and inexcusable for the other, which shows how far the two have drifted apart.
1adjectiveunderstandableFair or natural in the circumstances; easy to see the reason for.
“His anger was understandable after what they had put him through.”
synonyms: reasonable, natural, excusable
2adjectiveclearAble to be understood; clear.
“She rewrote the instructions in understandable English.”
synonyms: clear, intelligible, comprehensible
Understandable is one of English's own builds: Middle English understondable, understand plus -able, the suffix doing what it always does, marking something as able to be done. It has been in the language since the Middle Ages. The forgiving sense is the same idea one step further on, because following how somebody got there is most of what excusing them means.
Middle English
understondable
English, today
understandable
From Middle English understondable, equivalent to understand + -able.
Etymology from Wiktionary, CC BY-SA