硝子窓 (garasu-mado) means “glass window,” written with the poetic kanji 硝子 rather than the more common ガラス. In the song, the window reflects and blurs the boundaries between “you” and “me,” making this vivid image central to its emotional world.
"GLASS WINDOW" starts as a plea from someone going under. The singer asks to be pulled in by the hand, "ねえ お願いこの手を牽き寄せ" [hey, please, pull me in by this hand], and admits "このまま人波に溺れそうだわ" [I feel like I'm about to drown in this crowd like this]. The window of the title is where the whole song lives: first "硝子窓に滲むあなた" [you, blurring on the glass window], then later "硝子窓に映るあなたはわたし" [the you reflected in the glass window is me]. That turn is the clever part. The person she wanted to be rescued by turns out to be her own reflection, and "他人事では居られない" [I can't treat it as someone else's problem] makes it plain she can't hand her troubles to anyone else.
The middle of the song is surprisingly gentle advice. "弱さは負けじゃない" [weakness isn't losing] and "壊れたら直せばいいよ" [if it breaks, you can just fix it] are blunt, simple lines, and a voice slips in with "だから泣かないでくれよハニー" [so please don't cry, honey]. One phrase worth learning: "糸目は付けない" literally means not putting a limit thread on something, and is an idiom for spending without holding back, so "今日だけは総てに糸目は付けないの" means [just for today, I'm not holding back on anything]. "高速を飛ばして" [speed down the expressway] is the escape fantasy, all the way "悲しみの向こう側まで" [to the other side of sadness]. By the end she has stopped wanting to be fixed. She loves the world as "煩わしき愛おしきこの世界" [this annoying, lovable world], and herself as "いびつそのままで" [just as lopsided as I am].