This is the 817th most frequent Japanese word.
ガラス
Glass.
Here, 'ガラス' is describing the sheet of glass used as part of the window.
この窓のガラスはとても分厚いです。
The glass of this window is very thick.
In this sentence, 'ガラス' refers to the material, glass, used in objects like windows and glasses.
ガラスが割れる音が聞こえました。
I heard the sound of glass breaking.
In this case, 'ガラス' specifies the material that the cups are made of, emphasizing their transparency typical of glass products.