It was released for digital download and to streaming platforms on January 22, 2020 by Atlantic Records, as the lead single from Williams' debut album Petals for Armor.

[5] The song was written by Williams, Paramore touring musician Joseph Howard, and guitarist Taylor York.

Hayley Williams wrote "Simmer" about her personal experiences with specific types of abuse she has dealt with in her life.

It hangs in the air like a provocation before she finishes: '... is a quiet thing'", also noticing the lack of "distorted, thrashing guitar[s]"; instead, saying it is accented by watery harp and ominous vocal harmonies.

[4] The music video depicts Williams running naked through a forest at night, seemingly being chased by an unknown figure.