When the 90-Degree Rule is in effect, golfers are required to keep carts on the cart path until they are even with a golf ball on the golf course. Only then should the cart leave the path, turning sharply (90 degrees) to drive straight across to the golf ball. After playing the shot, the cart should be driven directly back to the cart path, then remain on the path until pulling even with another ball. In this manner, golfers have the convenience of carts but damage to the course is minimized.
The above picture depicts a golf hole and the location of two golf shots indicated by the #1 and #2 in the black circles. The two lines indicate how the driving style of a cart correctly follow the 90 degree rule (red line) and incorrectly follows the rule thus causing damage to the turf and ultimately poor lies (black line).
The 90-Degree Rule is permanenty in effect at many courses; at others, it will be put into effect following rains or when course conditions warrant. Look for signs near the first tee that might indicate whether the condition is in effect, or ask in the pro shop.
Even when the 90-Degree Rule is not in effect at a course, it's a good practice to follow because it helps maintain a healthier turf.