Spell check does not now, nor has it ever known what redware is. And it's really very simple and very very broad: pottery made with clay that has iron in it. Oh, neither the clay nor the pottery is always red. I suppose that amounts to a complication. The clay can be grey or tan or beige or any color that's remotely associated with iron and its various compounds.
Let's go back a bit. Clay is an earth that's origins are feldspathic rock. When weathering or other forces decompose the rock, it forms platelet shaped crystals that act like clay acts. Pure clay is white. One of the things that happens to pure clay is that it gets mixed with other minerals and here comes iron in iron's many forms. Please forgive me if this explanation is over simplified.
Jumping back, the pottery called redware can countless colors and has been woman- and man-made for as long as human kind has been making things (pottery, sculpture, buildings, etc.) from clay.