帆 means “sail,” the large piece of fabric that catches the wind and moves a boat. In the song, the singers join pieces of thin cloth together to raise a sail, creating a memorable metaphor for people working together and moving forward despite the difficult times ahead.
Sakanaction build this whole song on one plain object: a piece of cloth. It opens with 「僕たちは 薄い布だ」 (we are a thin cloth), 「折り目のない ただの布だ」 (just a plain cloth with no folds). Nothing has marked them yet. Shadows pass straight through without staining, and even sadness only gets them wet before they dry again. It is a quietly confident picture of being young.
Then the years arrive. The singer doesn't know whether 「時代の泥」 (the mud of the times) waiting ahead will stain them, and later he pictures 「時代のハサミ」 (the scissors of the times) cutting their sail to pieces. That sail matters: in the second verse they stitch their thin cloths together, 「帆を立てた」 (we raised a sail), so they won't miss the wind. His answer to the scissors is simple and a bit stubborn: 「また繋ぎ合わせるから」 (because we'll stitch it back together again).
The ending turns personal. He asks 「思い出しても許してくれるかい?」 (will you forgive me if I remember you?), and then circles the phrase 「変わらないこと」 (the thing that doesn't change) until it becomes the point. Learner tip: 変わらない is the negative of 変わる (to change), and you'll hear 繋ぎ合わせる (to join together) twice, a handy compound verb built from 繋ぐ (to connect) plus 合わせる (to put together).