From wiki – an excerpt:

Rzeznik was approached to write a song for the City of Angels soundtrack, and the end product was “Iris“. This song propelled the band to stardom, as it stayed on top of Billboard Hot 100 Airplay charts for a record-breaking 18 weeks and spent 4 weeks at No. 1 on Billboard’s Pop Songs chart. Later that year, it was nominated for three Grammys.

According to several interviews with Rzeznik, he was experiencing serious bouts of writer’s block when he was approached, and was on the verge of quitting the band days before he wrote the song.