I am no expert on Zen Buddhism, SOA, but I suspect the answer to your question "What is Reality in Zen" will vary somewhat depending on which Zen monk or master you are talking about.
Now having said that, the only Zen master that I myself know anything about when it comes to their notion of what reality is Dogen Kigen. I recall he lived in the first half of the 13th century and was the founder of the Soto school of Zen.
According to Dogen, everything is made of time. That is, time is the basic "substance" or nature of reality. So, for instance, the tree in your yard is made of time. The bird in the tree is made of time. The blue color of the bird is made of time. Even your self is made of time. Even good and evil are made of time.
Hence, for Dogen, all of the things that most of us call "beings" are fundamentally, basically made of time. Dogen thus uses a Japanese word or term for things -- which I don't remember -- that means "being-time". All things -- everything -- are units of being-time.
Now, one of the implications of the fact that everything is made of time is that everything flows. That is, nothing that exists is permanent.
I hope that helps. I don't really know a whole lot about what reality is in Zen.