This is a 20 meter spool of high-quality flourocarbon level line for your tenkara fishing. Cut your own lines to any length you desire. Multiple sections of line can also be connected when a longer line is called for. Chris Stewart has a useful page on level lines that shows his simple but unique rigging method.
Level lines are available in a range of weights. The larger the number, the heavier the line. Heavier lines will load the rod more deeply and carry more inertia, making them easier to cast - especially in windy conditions. Lighter lines will allow more delicate presentations of the fly and help prevent drag due to line sag. We typically recommend starting out with a slightly heavier line (#3.5 or #4) and experimenting with lighter lines as you progress.
You'll also need a spool to store your lines. These ultra-minimal, simple and inexpensive line cards work great for level lines. You can also use the Meiho spools or foam spools - either will slide onto your collapsed rod for easy carrying between fishing locations. The clip-on line winders we sell are great for furled lines but will kink your level line, so best to stick with the spools or cards.