It's when you open a publicly facing port and map (forward) it to a local port your machine. In this case, it's opened at the vpn provider's public gateway. Otherwise, it would typically be opened in your router instead.
You can then configure your torrent client to listen on that local port that the public port is forwarded to. I think generally the public and the local port are the same number when using VPN.
If you do that, then others have the ability to initiate a connection to you instead of only you being able to initiate the connection to somebody else.
When seeding/leeching to/from someone else, at least one of you needs a port open. So, if you always have one open, you allow yourself to connect to anyone on the network regardless if they have one open or not.
Sorry if I confused you more, I'm not that great at explaining.
Since people keep bringing up tauri, here's the comparison made in the README:
Dioxus vs Tauri
Tauri is a framework for building desktop (and soon, mobile) apps where your frontend is written in a web-based framework like React, Vue, Svelte, etc. Whenever you need to do native work, you can write Rust functions and call them from your frontend.
Natively Rust: Tauri's architecture limits your UI to either JavaScript or WebAssembly. With Dioxus, your Rust code is running natively on the user's machine, letting you do things like spawning threads, accessing the filesystem, without any IPC bridge. This drastically simplifies your app's architecture and makes it easier to build. You can build a Tauri app with Dioxus-Web as a frontend if you'd like.
Different scopes: Tauri needs to support JavaScript and its complex build tooling, limiting the scope of what you can do with it. Since Dioxus is exclusively focused on Rust, we're able to provide extra utilities like Server Functions, advanced bundling, and a native renderer.
Shared DNA: While Tauri and Dioxus are separate projects, they do share libraries like Tao and Wry: windowing and webview libraries maintained by the Tauri team.