This is the 10746th most frequent Turkish word.
dağ
Mountain.
In this case, 'dağda' is the locative form of 'dağ', describing the location being on the mountain.
Dağda güzel bir manzara var.
There is a beautiful view on the mountain.
In this sentence, 'dağ' is used nominatively to refer to a tall mountain.
Bu dağ çok yüksek.
This mountain is very high.
Here, 'dağa' is the dative form of 'dağ', indicating motion toward the mountain.