The Web 1.0 way of doing things has all of our code happening in succession—waiting for each line of code to complete before moving on to the next. Much like building a house, the foundation must be complete before the walls can be built, then the walls must be complete before the roof is built.
With Ext, we can easily start working on the roof of our house before the foundation has even been thought about. Imagine the roof of our house is being built in a factory, while at the same time we are building the foundation, then the walls, and we come in when all of this is done and set the roof that has already been built on top of it all.
This introduces some things we're not use to having to cope with, such as the roof being complete before the walls are done. No longer are we forced to take a line-by-line approach to web development.
Ext helps us out by giving us events and handlers to which we can attach our functionality. We can set up an event that waits around, watching to see when the walls of the house are built, and then sets the roof on top once this has happened.
This method of thinking about web pages is hard for most people who have grown up in web development. But it won't be long before you are an expert at it.