That resistor is a "pull-up" resistor. Designers use pull-up (or pull-down) resistors so that the state of the signal, in this case BUTTON_TO_AP_HOLD_KEY_L, is in a known state; in this case @ 1.8V (or "high" or "1") unless the line is brought down to 0V.
The line designator explains what it does...button (hold key) to application processor with an active "Low" signal. Basically, if the Power button is pressed, it's signal will go Low and it will notify both the PMIC and the CPU of this for appropriate downstream actions. Just follow it through the schematic to see where it goes.
Think of the circuit like this:
Now imagine that you cut a line or remove components and you will have a better understanding of what the impact is.