It's a US-specific system for alerting people to missing children. It's named after Amber Hagerman⁰, a 9-year-old who was abducted and murdered in the '90s. On smartphones they can appear as (very loud) notifications with details about the missing child, potential abductors, vehicle, etc. You'll also see the same info on electronic message boards along the motorway.
[0] https://en.wikipedia.org/wiki/Amber_alert#Namesake