What does it mean when ISIS claims responsibility for an attack?

A defaced Islamic State flag is emblazoned on a wall in Tikrit, Iraq, in 2015. ISIS will generally claim responsibility for an attack within one day, though it can sometimes take longer.
A defaced Islamic State flag is emblazoned on a wall in Tikrit, Iraq, in 2015. ISIS will generally claim responsibility for an attack within one day, though it can sometimes take longer.