RESET in Microcontroller
As it says, it is an I/O, so it's both. Which is almost always the case with reset pins on every MCUs out there. They are almost always active low as well.
When the MCU is resetting, it will pull this pin low. You can also force a MCU reset from an external source by pulling it low. Common EMC practice is to provide an external pull-up there (~10k) to prevent accidental resets, though most modern MCUs have built-in pull-up.
Some MCUs may be picky about the value of a capacitor placed on the reset pin too - always check what value the manufacturer recommends to place there.