Flask is a widely used, lightweight Python framework ideal for quickly building web applications, often running on the default port 5000. However, conflicts can arise if this port is already in use, such as by another Flask application or the Airplay Receiver on Macs with Monterey or later versions. To resolve such issues, developers can either stop the existing application or switch to a different port using commands like `$ flask run -h localhost -p 3000`. Additionally, running Flask applications with host settings such as `0.0.0.0` or customizing the host and port, like `127.0.0.9:8080`, can help make the application accessible across a network or resolve authorization errors. The text also suggests turning off Airplay Receiver if it interferes with the Flask application. For those interested in integrating features like email or calendar functionalities, Nylas offers tools and a free trial to enhance Flask applications.