Here, a speaker tries to describe a love big enough to leave the ordinary world behind. It begins with a castle of glass and ice, a striking image of something beautiful but delicate. The speaker plays down the fragility with a small joke: call it an igloo if you like, but it melts "real slow." That suggests hope that the love can last, even if it isn't indestructible.
From there the poem grows more daring. The speaker wants to ignore other people's approval, admits to being "susceptible" to this one person, and imagines drifting off like shooting stars toward Orion's belt to learn how love is "actually spelled." The tone is playful, a little dreamy, and openly romantic. The fireflies in a jar are a nice twist, since they hint at being trapped as well as glowing. The final lines turn defiant: with this person, years feel like hours, and nobody can hold the speaker back. Overall, the mood is giddy, bold, and tender.