Port forwarding is a networking technique that allows devices outside a private network to access applications running inside by mapping an external port to an internal one, effectively "opening a door" within the network. It serves various purposes such as remote management, web development collaboration, and online gaming by selectively granting access to internal applications or services. Port forwarding works by routing incoming requests from the internet to specific devices on the private network using router rules, which can be complex to set up and maintain, posing security risks if not configured properly. However, alternative solutions like ngrok offer a more secure and user-friendly way to expose internal applications to the internet, utilizing secure tunnels and encryption to protect communication and data transmission.