Readability refers to the ease of comprehending your poster. Typical "readability" indexes stress using short, familiar words and short, simple sentences. But a poster may need to express complex or difficult material quickly, and so short, easy words and sentences may not work for your poster.
A good way to check readability is to read your poster aloud. If you can "hear" your poster and it makes sense, you probably have a good start. If you stumble over any sentences, revise those. Then ask a few people typical of your target audience to read aloud a sample layout of your poster while you listen. Note when they stumble over sentences or puzzle over missing connections between sentences. Watch how they move from chunk to chunk of the poster and consider rearranging to enhance the flow of ideas. The more work you do to insure that readers can comprehend your ideas, the more successful your poster will be.