You may be used to eating spinach leaves in salads, or sautéed. In Japan spinach is rarely eaten raw. The most common way to eat spinach is to blanch it briefly. You may lose some nutrients when you do this, but it's more than made up for I think by the fact that you can eat a whole lot more spinach than in a salad or so.

In the U.S. and Europe, it's probably easier these days to buy ready-washed bags of the leaves only. This is a bit of a shame really, because spinach stalks and roots have a different texture which adds interest. In any case, the instructions here assume that you are dealing with the leaves only.