Most MCUs have a reset pin. This pin should have a high voltage normally, and it is connected to ground when reset is needed.
The reset line is usually “pulled up” to the output of a regulator, the same as the power pin of an MCU. This means that the reset pin is connected to a resistor, and the other end of the resistor connects to the regulated voltage node. The value of the resistor should be 10kOhm or so.
Because the reset pin is a very low load pin, it means that normally, it has the same voltage as the output of the regulator.
The reset node can be “grounded” by two means. One is to use a jumper (a 1x2 0.1 inch SIP jumper) to connect (short) it to ground. This is a useful mechanism to allow a user to reset an embedded controller manually.
The reset node can also be grounded by a “supervisor” chip. A supervisor chip monitors the output of a regulator. If that voltage drops below a certain threshold, then the reset line will be grounded. The threshold is usually specified so that it is still a “healthy” supply voltage to the MCU.
The purpose of the supervisor chip is to prevent an MCU from exposure to a voltage that is not high enough for reliable operation. When the reset line is asserted (shorted to ground), an MCU reverts to a “safe state” and does not operate in any way. This is better than having an MCU exposed to a low voltage and “act drunk” instead!