Black has become secure by having enough spaces (or, eyes) where white won't play.
Then white played G2.
Look what happens if black answers at F1 ( or G1) before you find the right answer.
If white can capture two black stones on the bottom side, she can live easily.
Your stones were in Atari. So you escaped.
At the next moment, your entire stones disappeared!
This tragedy is called "Oiotoshi".
You should have escaped *before* you were in Atari.
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