Web usability is a fancy term meaning “Is a given web site easy to use and understand?”
Anyone that contributes to a web site in any capacity (programmer, designer, writer, project manager, business owner) should get a crash course in web usability by reading the short, excellent book Don't Make Me Think.
The book also has a companion chapter on mobile usability that is a must-read for mobile application developers.
Once you've read that first book, follow it up with Rocket Surgery Made Easy. This second book will teach you how to perform usability testing on the cheap so that you can quickly find and fix usability problems.
I strongly recommend both of these books.
They are a quick, light, easy read and will immediately improve your web development efforts.