It's the brown-out recovery voltage. Brown-out is the voltage at which the under-voltage-lockout will take effect and brown-in is the voltage that the under-voltage-lockout will cease to be in effect.
e.g. brown-out voltage may be 60V and brown-in might be 72V; if the voltage drops below 60V the device will stop operating and won't start operating again until the input voltage exceeds 72V.