I just got a Nishiki cross bike and it has a Shimano Revoshift 7 speed. The bike is about 3 years old or so and did not come with a manual and I am a biking novice. The shifting I understand. The left handlebar has a Revoshift friction handle with a "+" and "-" setting. What is the purpose of this and what is the best setting? I noticed that if it is set for least friction the chain came off the gears.
A friction shifter allows you to continuously move the derailleur. The point of this on a front shifter (the left) is to trim the front derailleur. Trimming the front derailleur is when you tweak the position of the front derailleur to account for the change in chain angle with shifting, e.g. eliminate the chain rubbing on the cage of the derailleur.
There isn't a "best" setting -- you'll adjust it depending on which gear combination you're in.