Ratio of how big the piece looks in that specific part of an instruction to how big it is irl. If it says 1:1, you can just place the needed piece on the page of instruction, and size will fit perfectly. Usually used for rods and other smooth things, size of which isn't inherently obvious from just one axonometry(normal bricks have studs, technic beams have holes, and you can count those to estimate actual size, but technic rods don't, and thus it's sometimes hard to tell how long rod is on instruction.