Web usability is an approach to make a web site easy to use for a user, without the requirement that any specialized training is undertaken or any special manual is read. Users should be able to intuitively determine the actions they need or can perform on the web page s, e.g., press a button to perform some action.
Some broad goals of usability could be: present the information to the user in a clear and concise way, give the correct choices to the users in an obvious way, remove any ambiguity regarding the consequences of an action (e.g. clicking on delete/remove/purchase), place important items in an appropriate area on a web page or a web application.
When you designed your web site, you want to promote it everywhere with big bold letters saying, "Hello everybody! Come and look at my web site! It is just great!" When you submit your web site to forums for web site reviews, you may write, "What do you think of my web site?"
This is the big mistake to ask someone to look at your web site. There is never a single answer. To understand if your web site meets its usability requirements, ask people to try it out. They should be able to answer the following questions: what is the purpose of this web site? what can I do here? what needs does it fulfill?