When the input voltage of a load switch IC drops below the internal threshold, the undervoltage lockout (UVLO) function turns off its output to ensure system stability. This function comes in handy because system operation might become unstable if the voltage supplied to a constituent IC or circuit drops. Undervoltage lockout has a hysteresis. When the input voltage rises back above the recovery threshold, the output automatically turns back on.
The following links also provide descriptions of undervoltage lockout:
Application note: Basics of Load Switch ICs
FAQ: Does the undervoltage lockout function of the load switch IC provide a hysteresis?