Grade is a number, expressed as a percentage, which shows how many feet per 100 feet the road rises or falls.
So an 8% grade falls (or rises) 8 feet every 100 feet of roadway.
Now, some rules of thumb:
1. 6% down grade, particularly for any long time, is all you want to pull(see my sig) with a big rig, unless you have an exhaust brake.
2. Anything over 6%, slow down to about 25 MPH, put your auto (or stick) into 2nd gear, and go down SLOW. Get on and off the brakes so they don't get real hot....if you're pulling a load.
They have a small stretch of 10% grade coming down to the Peace River in British Columbia and it is scary with a rig on the back of the truck.
3. If you're not pulling anything, screw the grade. You won't really care.....