Torque, in this instance, is the measure of how tight a nut or bolt needs to be. The great dirt bike writer Ed Hertfelder once opined that there were three torque levels: White knuckles, grit your teeth, and break wind. This is usually followed by a fourth—the bolt snapping off.
You can be more precise with a torque wrench—a special wrench that measures how much rotational force you’re applying to a bolt. There are two types: The beam type with a pointer indicating how tight the nut or bolt is, and the click type which uses a calibrated clutch (usually set by turning the handle to the desired torque setting). Both work well, although the beam type is usually less expensive.
Torque is measured in foot-pounds or newton-meters. Really small bolts are measured in inch-pounds or newton-centimeters—don’t confuse the two. When you use a torque wrench, be sure the nut or bolt is clean. The specs will tell you whether it should be dry or lubricated.
With critical bolts (like engine connecting rods) there should be no guesswork—refer to a detailed shop manual. But for other fasteners (handlebars, axle nuts, switches) some general guidelines are in the following entry. Note that these are for the bolt-shaft diameter, not the measurement across the flats on the bolt head.