Instant rice (also called "rapid cooking" or "precooked" rice) is simply rice that has been fully cooked before you buy it. Once this rice has been precooked in the factory, it's dried to a low moisture level (usually 12% or less) so that it can be packaged and be shelf stable. Because it's been precooked, instant rice doesn't need to be fully cooked again, which is the reason for its short cooking time right out of the package. Yet, while it may be a convenient option, instant rice is usually more expensive than regular rice that needs to be fully cooked.
Both white rice and brown rice can be precooked and can be featured as "instant rice" products. Since the nutritional quality of the instant rice that emerges out of the cooking process depends on the nutritional quality of the rice going into that cooking process, instant brown rice is a much better choice than instant white rice for your Healthiest Way of Eating.
I like to prepare my rice from scratch using regular brown rice as I prefer its flavor, smell and texture over that of instant rice; yet, I do understand that the difference in cooking time (a few minutes for instant rice compared to about 45 minutes for rice prepared from scratch) may be important for some people. Since there is little nutritional difference between instant rice and rice prepared from scratch, I would support the use of instant rice when necessary. But I would make a point to purchase high-quality instant rice like organic, whole grain, instant brown rice rather than instant white rice or even conventionally grown brown rice. You'll still get the convenience but you won't have to sacrifice as much nutritional value, and with an organic instant rice, you won't have to risk unnecessary exposure to contaminants.