Help understanding resistor placement in 741 voltage reference circuit
The two resistors you mention are potentiometers, not simple resistors.

A potentiometer has three terminals: the two ends of the resistance element, and a moving contact that slides along the resistance element.  The resistance between the moving contact and either end contact will vary as the contact is moved. The arrow pointing into the side of the resistor box represents the moving contact of the pot.

A common application of a potentiometer (or "pot") is as a volume control.

In your circuits, the pots are used to set the threshold voltage - the voltage that the circuit compares the input voltage against.

By the way, the 741 is a very old op-amp, and requires + and - power supplies, and can't get its output closer than 3 volts or so to the power supplies.  For this application an analog comparator would be much more appropriate.