The phrase "plain rice" means that rice is cooked without any added flavors or ingredients. It refers to the basic and simple way of preparing rice. Full definition
So, unless you're eating a plate of plain rice first thing in the morning, the type of rice you eat is irrelevant and will digest slowly anyway. (fitnessandpower.com)
This video shows how to throw them together and different ideas to mix into the rice, but you can make them with just plain rice too. (breastfeedbabywearclothdiaper.com)
I've never actually tried to cook plain rice in the oven before, but apparently it works. (treehugger.com)