White soy sauce is a pale, straw-yellow color. When soybeans oxidize they become dark, so to make white soy sauce you use mostly wheat and only a little soy. To make the palest shiro they could, third-generation Japanese company Nitto Jozo skips the soybeans altogether and uses 100% wheat.
Why would you want white soy sauce? It's more delicate in flavor than other soy sauces, with a subtle sweetness to balance the salty notes—though don't get me wrong, it still has plenty of salty kick. It's traditionally used in Japan for dishes where you want to add the oomph of soy sauce but you don't want the color of regular soy sauce to darken the food, like in a Japanese egg custard. The gentler flavor also makes it a good match for lighter dishes, like sashimi. But don't stop there. It's great for adding a subtle umami kick to everything from broths to tacos to vinaigrettes.
Finally, a note about the name. Since this sauce doesn’t contain soy, Japanese law dictates that it can’t be called “shoyu” (the Japanese word for "soy sauce") so instead it bears the name “tamari.” In the US, the word "tamari" is sometimes associated with gluten free soy sauce, but since this is made of wheat that's not the case here.