Some people (almost all people in some way) dream of an idealized world (or "heaven" or "enlightenment" or a "purified society after the revolution comes" ... whatever) that is always good by our little human standards ... candy cane trees and ice cream mountains. Or, they feel lack between how the world "is" and how they wish it "should be" in their ideals. At least, they dream of some state much better than the present state. In contrast, this world of ours is less than ideal. That is an "idealistic" view. There is also a sense in most religions of some "ideal" world that is the world of the spirit, which is the world we need to get to by escaping this world of the "flesh".
On the other hand, some other people think of this universe as just blind processes, dead matter that happened to come alive as us, going no place in particular. (I really abbreviate the description ... but this is generally a materialistic view of the world). Although seemingly dispassionate and "coldly objective" about the world, this view will often cross the line into asserting that the world is "meaningless" or "pointless" or "survival-of-the-fittest cruel" or just "we are born, we work, we die" ... some such bleak thing. He also sometimes uses "material" to mean the "world of the flesh, this sometimes disappointing and hard life" as opposed to the above idealized "world of the spirit" found in most religions.
Both those views tend to judge that there is something lacking in the present state.
However, Buddhism is an existentialist way of being in and as this life-world-just-as-it-is
, meaning the world and this life before we impose our judgments and dreams upon it. We neither judge the world lacking in comparison to another ideal world, nor do we judge it cold and pointless and hopeless. We just let the world be as it is, and we go with the flow ... to such a degree that we can no longer see perhaps the divisions between ourselves and the world in the flowing. In that way, as Nishijima describes it, it swallows whole both materialism and idealism by finding this world, just going where it goes, to be ideally just what it is. And that way of seeing beyond "beautiful" or "ugly", "peace" and "war" is .... pretty darn Beautiful and Peaceful! Material and Ideal merge into each other and are transcended. This is Nishijima's view of Buddhist "realism", his third philosophy.
However, theory alone is not enough. More than words describing this "realistic" perspective, we must actually taste it in the practice-experience of Zazen. So, Zazen is the pure action whereby we actually experience this being of reality.
Something like that.