Magic roots are the roots of the Magic tree which is obtained with the Farming skill. Level 75 farmers may plant a magic seed in a Tree patch, and once the tree has grown, the farmer may use an axe to chop the tree down to a stump. As with all player-grown trees, if you do not use a spade to dig up the roots, the tree will re-spawn and can be cut down again. The farmer may then use a spade on the stump to dig up the tree roots, clearing the patch for another tree, and putting the Magic root into the farmer's inventory. As with all tree roots, magic roots may be used in a compost bin to make supercompost. However, magic roots may be spun into magic string (instead of crossbow string like other roots) by using the magic roots on a spinning wheel, with level 19 Crafting, which is a component to make an Amulet of nature. Magic roots are also a component in the Herblore skill to make Super strong anti-poison potions.
The fact that magic roots are the only roots not currently used in the Fletching skill may be evidence that a Dragon crossbow will be released in the future, for which magic roots would have to be used for the crossbow string.