G'day Brad03,
2 wheel high is for on road driving, meaning bitumen or other high friction surface. You can leave the front hubs unlocked or in the "free" position. When 2 wheel high is selected, drive is through the rear wheels only. Normal road speeds can be maintained in 2 wheel high.
4 wheel high is for driving on slippery surfaces like mud, snow, ice or gravel roads. Before selecting 4 wheel high ensure the front hubs are in the "locked" position. When 4 wheel high is selected, drive is through the front and rear wheels. Normal road speeds can be maintained in 4 wheel high.
4 wheel low is for very difficult terrain where maximum traction and torque is required. Before selecting 4 wheel low ensure you have come to a complete stop then move the lever into 4 wheel low. The same applies when moving the lever out of 4 wheel low. Once again ensure the front hubs are in the "locked" position. Road speed is very much reduced in 4 wheel low. 4 wheel low should only be used as long as is necessary.
When selecting 4 wheel high or 4 wheel low it is critical the front hubs are in the "locked" position. If not, drive will only be through the rear wheels even though the "4WD" light is illuminated on the dash. With the hubs in the "free" position the front wheels are disconnected from the axles therefore, no drive.
Finally, never, never, never ever drive on bitumen or other high friction surface in 4WD as expensive damage will result to the drivetrain. At the very least a rebuild of the transfer case.
Hope this helps you out. Suggest that your few first 4WD trips be in the company of experienced 4 wheel drivers, you'll learn a lot from them. Failing that look at joining a 4WD club.
Cheers
Footloose