This is the 16470th most frequent Turkish word.
yağan
"Yağan" means "falling" or "that which falls," typically used for precipitation like rain or snow.
Here, 'yağan' is used to describe the rain that is actively falling at the moment.
Yağan yağmur şehri serinletti.
The rain that was falling cooled down the city.
In this sentence, 'yağan' describes the snow as an action that generally occurs in the mentioned subject 'mountains' during winter.
Kar yağan dağlar kışın çok güzel görünür.
The mountains where snow falls look very beautiful in winter.
'Yağan' here is metaphorically used for falling leaves, indicating a poetic expression describing leaves descending in fall.