This code is creating a server using the “express” library. The server listens for a request on the route ‘/’, and when it receives one it sends back the text “Hello World”. The server is also listening on port 3000.

There is also a connection being made to a MongoDB database using the “mongodb” library. This database is running on the same machine as the server, and the database is called “url-shortener”.

There is a function called “createShortLink” that is being used to create a short link for a given URL. It does this by connecting to the MongoDB database, and generating a random string to use as the short link. It checks to make sure the short link is not already in use, and if it is not, it inserts the new link into the database.

There is also a function called “generateRandomString” that is being used to generate a random string of 6 characters.

Finally, there is a route called ‘/shorten’ that is being set up to handle a POST request. When it receives a request, it takes the “url” from the request body and creates a short link for it using the “createShortLink” function. It then sends back the short link as the response.