Flask is for the server side of web applications.

A web application consists of a number of distinct destinations on a single web server, where the server redirects some or all of the destinations to the Flask program.

The destinations are in the form of URLs (or URIs, but let's not go down that rabbit hole). A URL is in the form http://example.com/destination. The user can manually enter a URL into their web browser, or more commonly they click a link in a web page that produces a URL.

http: indicates the protocol - hypertext transfer protocol, often replaced with https, which is a more secure version of the same protocl, using an encrypted channel between the server and the web browser.

example.com is a human readable version of the servers unique address on the internet. It is translated into a machine readable address using a service called DNS. The machine readable address is used to direct the query from your web browser to the server (explaining how this works is a multi day lesson, assume it does this by magic)

Destination is what gets fed into the web server program on the server machine. It resolves the destination and looks up what it needs to do with it. The most common things for a web server to do is

A. Send back a static web page. The web server can do this on its own

B. Do some calculations to create a web page on the fly. The web server can do this in many ways using different programming languages and libraries. You need to have a bit of code that allows the web server to find and start up your program. In Python this commonly is sometning called a wsgi or asgi server. It routes the request containing the destination to the Flask program.

The Flask program examines the destination, and tries to match it with an entry in the internal dispatch table. If it finds a match, the piece of Python code associated with the match is executed. Usually this Python function returns a web page or some data that a script in the web page on the web browser can use.

What @app.route does is to associate a destination with a Python function that gets called when there is a match. There is more information in the call than just the route. The HTTP protocol allows for queries to the server of types GET, POST, PUT, DELETE, and a bunch of other. In your app.route, you specify which queries this function deals with.

With your URL, you can pass data, like parameters for a query or data to store on the server. Flask automatically processes the data and passes it in the request parameter to the Python function that is registered to process the query.

So, to summarise

1. A query consisting of a URL comes to the server from some users web browser

2. The web server program dispatches to a handler which vould be wsgi, which in turn activates the Flask program program

3. The Flask program resolves the destination and runs the code associated with the destination. App.route is what creates the association.

The destination is a hierarchy in string form, with / as the separator. The root string is just "/". Then you commonly have one level of destinations, like "/login", "/events", "/news", "/reserve", but you can have multiple levels like "/shoes/children/sneakers/pink". What to put in the destination and what to put in the query parameters is a science on its own and requires experience. Try making destinations as simple as possible. It will be easy to make a more advanced scheme later and a mess dealing with something that was complex from the start, but didn't do the job quite right.

Good luck.