Readability is the ease at which a reader can read, comprehend, and—this is the most important part—submit to your story. Readability is doing everything you can do within your power as an author to ensure nothing, i.e., nebulous words, weak sentence structure, bad grammar, pronoun confusion, crude punctuation, continuity issues, no real plot, feeble plot flow, insufficient character development, poor pacing, etc., in your manuscript makes a reader stop, question, and/or leave before he or she really wants to.
I, as your editor, act as a reader, trying to catch everything and anything that makes me cease reading (even for a second). If something makes me stop and pulls me out of the story, I then put on my editing hat to determine (1) what could be the reason I stopped reading and (2) how can it easily be fixed without trampling on an author’s style and tone.