A DC-DC converter uses a voltage feedback to regulate the output.
Normally, a DC-DC converter is used in Constant Voltage mode. With negative side of the load connected to Ground and the positive side connected to the converter output.
Then a voltage divider connected across the load, connecting to the voltage sense of the chip so to maintain a constant output voltage.
For Constant Current mode, you do away with the voltage divider and use a single resistor for the feedback.
You do this by connecting one end to Ground, and the other end to the negative side of the load as well as the feedback of the chip.
Then the positive side of the load to the converter output.
The images below show the basic differences between Constant Voltage mode(left) and Constant Current mode(right).