http://breezewiki.vernccvbvyi5qhfzyqengccj7lkove6bjot2xhh5kajhwvidqafczrad.onion/minecraft/wiki/Redstone_Torch
Effectively, a redstone torch inverts the signal applied to its attachment block: power level 0 is changed to 15 and power levels 1 to 15 are changed to 0 (for an alternative that produces a greater range of output power levels, consider a redstone comparator in subtraction mode). Walls, fences, glass, slabs, hoppers, and stairs cannot be powered so redstone torches attached to them cannot be deactivated.