This is the 1520th most frequent Turkish word.
giderken
"While going" or "on the way".
Here, 'giderken' is used to indicate an action ('going') that occurs simultaneously with another event ('it started to rain').
Okula giderken yağmur başladı.
While going to school, it started to rain.
In this sentence, 'giderken' specifies a state ('going') during which another action ('I saw her') occurred.
Evine giderken onu gördüm.
I saw her while she was going to her house.
The word 'giderken' here conveys the ongoing action of movement ('going to the market') during which an observable state ('she was wearing a green sweater') is described.