This song is an autobiographical account of Sigur Ros' first listen to the finished mix of their debut album, Von, which only sold about 40 copies upon release. Bassist Georg Holm recalled to Uncut:
"I remember going to our local cafe and asking the guys there if they'd put the record on, and with every song we got more depressed, because we thought it basically sounded like s--t. We hated it! The lyrics to the song 'Agaetis Byrjun' are actually about that day."
Sigur Ros first used their made-up Hopelandic language during the last section of this track along with another of the album's songs "Olsen Olsen."