Supply Supervisor ICs will monitor a supply rail for over- or under-voltage conditions. The left figure shows a typical supply supervisor that checks for supply under-voltage conditions.
When the supply rail drops below a certain level, the IC will initiate a reset signal. This signal can then be used to reset the MCU that is fed by the same supply rail, to avoid any data corruption that could occur due to too low MCU supply voltage.
When the supply voltage recovers to normal levels, the supervisor IC will remove the reset signal with some delay, to ensure correct MCU initialization.
Some supervisor ICs have a manual reset pin as well, that can be used to reset the MCU via an external reset switch.
Selection criteria for supervisor ICs are voltage threshold, reset delay time, open-drain or push-pull output, and options like manual reset.