When something declines, it goes downhill. If you stop working out, your health may decline. If you want it to improve, you should start climbing inclines instead of coasting down declines.
The verb decline means refuse to accept, but the noun decline means a downward slope (the opposite of an incline), or a decrease in quality. At the gym, the treadmill has an incline button to control how steep your climb is. Sadly, there's no button for decline, which would be much easier. Both words contain cline, or "to bend." The prefix tells you which way the bend goes. Decline implies a going, or turning, down.