That's Right, Indeed
If you become aware of something to an extent you'll end up saying I see or That's right! But you won't see if you didn't become aware enough. So it's that extent of understanding you're trying to reach before you can chime in with an "Ahh, indeed."
The readings for this word are both kun'yomi readings. The reading for 成 is from 成る and it keeps the る part. It's just hidden in the kanji. If you can remember that, you'll know this reading!
I see! I can send this letter without anyone finding out.