A transfer case is the mechanical component that connects the final drive output of the transmission to the front and rear wheels on a 4WD or AWD vehicle.
These can range from simple mechanical devices to highly sophisticated components that dial in a prescribed amount of power to the front and rear wheels. As a result of this, you don't do too much servicing of a transfer case outside of fluid changes. So if you suspect a noise from the transfer case, replace the fluid with the correct fluid as dictated by the service manual and recheck for the noise. If the noise goes away, call it a win and move on. If not, you might have to replace or rebuild the transfer case. As I said, these run the gamut from simple mechanical devices to highly sophisticated, precision-engineered components. You really don't want to get started here unless you know what you're getting into.