Tinder is a substance that will catch a spark and form a "coal" or ember of glowing material. One of the best materials for this is "char cloth" which is cotton cloth that has been heated without air until it is darkened or black.
A tinder tube is a way to create such a material automatically, as well as easily and safely extinguish the ember. Whenever a lighted coal on the end of a cord of suitable material is extinguished by sliding a metal tube over it, char is created for the next use. 100% cotton cord is an excellent material.
To use it with a ferrocerium (sparking) rod you can slide the tube back far enough away from the charred end of the cord so that the char can be held against your "striker" in front of it as it is scraped against the rod where it will catch the spark from the ferrocerium. If using flint and steel, hold it right in front of the flint. Held correctly it will usually catch the first spark.
Of course it will also light from a small coal such as formed in a "fire piston" and form a more durable coal. ( A "fire piston" compresses air to light a small piece of tinder, instead of using exhaustable materials such as ferrocerium). The sun focused with a magnifying glass or flashlite reflector will also light it easily.
To extinguish the coal and preserve the char for next time, the tube is just slid right over the glowing coal.
A metal (brass is good) tube about 2 in. long, and 1/4 inch in inside diameter, with a length of cotton cord such as Wright's 6/32" (The dimension printed on the package; I would call it "3/16 in.") 100% cotton filler cord inserted through the tube, works really well. 10 yd. lengths of Wright's 100% cotton cord is available (now, 4/2010) in Wal-Mart's notions dept. or you may find a source at www.wrights.com. Beware some so-called "cotton cords" are not pure cotton, and won't make good char or even maintain an ember.
1. A knot on the back end of the cord will prevent the tube from slipping off when using, as well as help prevent unraveling.
2. A small hole or cross-slot near the back end of the tube to insert a safety pin through the cord will prevent losing the tube from the front end when not in use.
3. The whole cord can have a slip-knot to make a large loop of the cord to temporarily hang around the neck, etc. But be sure you don't get it sweaty or wet in any way. It can be doubled for long lengths such as 10 ft. This will make a loop that hangs about 20 in. from the front of the neck after the knots, neck-width, etc.