An ESU (Electronic Software Update) affects a set of user objects (application programs). Objects in an ESU are grouped by dependencies. The changes you get to a specific object are cumulative so that you always have the most recent version of the object when you download a fix in a certain area. They are also pathcode specific so they can be tested before being moved to the production set of objects.
Typically you download an ESU from the internet, apply it to the development pathcode, build it into an update package, deploy it to a test machine and then test it. You do all of this without impacting your production set of objects. These fixes should never reach production until they have been fully tested and approved.