A web application is anything that uses a Browser (Firefox, Chrome, etc) to interact with a server (the computer which holds the content you're trying to access) over the internet. Facebook is a web application, as is allDeaf.
With that in mind, web application architecture would be how a web application is structured to deliver it's functionality to a browser and meet the needs of both the end user and the organization providing the service. As you might imagine, the needs of a banking web app might be much different than a social media web app, or a simple browser game. These business and user needs influence how the application is built and what components are needed for it to function in a reliable, safe, and user-friendly manner. The collective result of all these considerations is the web application architecture.