“Simmer” marks Hayley Williams‘ debut as a solo artist. The track, with a groovy and dark instrumental, deals with bottling anger and controlling it. It follows Hayley as she goes through feelings of anxiety, desperation, anger, and compassion. Lyrically, the song ends with Hayley finding peace and comfort in her femininity, expressed metaphorically as wearing flower petals as armor.

Though the project is completely separate from her band Paramore, the song is co-written by Paramore guitarist Taylor York and touring bassist Joey Howard. York has also produced the track.