Better understanding of Drupal behaviors
In short, the advantage of Behaviors over document.ready() is that they are automatically re-applied to any content loaded through AJAX. mymodule is your namespace, which needs to be unique. context is the part of the page for which this applies, for example a part of a form that has been updated with AJAX. You can attach multiple behaviors, but I think you need to use a unique name (mymodule) for each of those.

Have a look at the following resources for more information:


Drupal behaviors
Drupal Javascript API
Changes in the Drupal JS API for D7