Well, it kind of depends on your interpretation. Billie Eilish once stated that songs are meant for the listener's interpretation and that artists shouldn't try to create one for the listener because every listener is impacted differently and has their own meaning for the song.

If you're curious to know my interpretation, here it is. So, if you watch the music video, you will see that most of the time they are in a glass box and every so often that glass box gets bigger and it also gets smaller. Every so often, above this glass box is a storm cloud that they can't escape because of the glass box. When I watched this video, it reminded me of what depression feels like. It feels like you're trapped in something that you can't escape. When I look at the lyrics, “Isn’t it lovely, all alone? Heart made of glass, my mind of stone. Tear me to pieces skin to bone. Hello, welcome home,” I get reminded of what this depression feels like. For instance, we take “Isn't it lovely, all alone?” I see that lyric as a sarcastic remark. It's not at all lovely when you feel like you're alone and the only one facing a specific problem. “Heart made of glass, my mind of stone” is basically saying that you've had your heart broken so many times that it might as well be made of glass, but your mind is made of stone because after all of those heartbreaks, you still showed willpower and determination to go on and this shows that you're a mentally strong person. “Tear me to pieces skin to bone” symbolises how it feels when something bad happens. The pain feels as if you're being torn apart from yourself. “Hello, welcome home” is lowkey the hardest hitting lyric in the entire song. It basically symbolises that moment when you thought you were getting better but then reality sets back in. It represents that moment when you thought you've found happiness but then the sadness quickly sets back in and despite whatever made you feel good for that short period of time, that sadness got even stronger due to the belief that you will never again find true happiness.

I could go on and describe how each lyric outside the chorus hits me, but that would take too long and I don't want to ruin anyone's interpretation of the song. Like I said, everybody interprets a song differently. Don't interpret a song one way just because somebody else interprets it that way. Listen to the lyrics, watch the music video, and interpret it for yourself. Find your own meaning because your meaning is the only one that matters in your life.