When a circuit powers up there’s all kinds of noise and ripples in the power supply. Many microcontrollers have a power-on-reset feature that keeps them reset until the power has stabilized. Without the reset, the chip might start running a program only to get hit by dip in voltage that freezes or resets the chip uncontrollably.
Brown-out-detect is similar. It resets the chip if there’s a dip in the power supply that could send the chip skidding off the road and into a ditch.
Both features are usually combined into one microcontroller pin. The pin is usually connected to the circuit power supply through an external resistor. You might recognize this feature on your favorite chips: ATMEGAs (RESET), MSP430s (RST), PIC (MCLR), and most others.
Microchip writes about how to use POR and BOD devices:
This application note discusses what microcontroller supervisory devices are, why they are needed and some factors to consider when choosing one. Supervisory devices is a broad term that encompasses POR (power on reset) devices, BOD (brown-out detect) devices and watchdog timer devices. This application note will cover supervisor devices with POR and BOD functions only.