What is plain flour?

All-purpose flour is most likely considered plain flour

Like Glinda said, all-purpose is probably what is meant, rather than cake flour or soy flour or whole wheat flour, etc.

Guessing your from the uk plain flour is as wondergirl says all purpose flour but with no raising agent, that's why we have self raising with is basically plain flour with a added raising agent.

Plain flour is just all purpose flour, which means it doesn't contain a raising agent. It is obtained by grinding and sifting the meal of cereal grain.

Another part of the definition of "plain" flour is that it is flour with a medium gluten content, thus is suitable for a variety of purposes, as opposed to bread flour (high gluten) and cake flour (low gluten).
Different areas in the United States have different preferences in this area. The South notably prefers a low gluten flour for most purposes, and southern brands of all purpose flour (like White Lily) tend to have a lower gluten content than other parts of the country.
In addition, unbleached all purpose flour tends to have a higher gluten content than bleached, and is therefore more suitable for yeast breads.
Incidentally, if you're shopping for the whole wheat versions of these flours, you'll frequently find the high gluten flour labeled "hard" wheat and the low gluten flour labeled "soft" wheat.