ngrok is reomote port forwarding. Basically, your computer is on a network, say a home, corporate or public network where the router has a public IP from your ISP, but within the network, the router and each device connected to it have private IPs. Which is good. You can to the internet but the internet doesn't get to you.
And that is sometimes a problem e.g. when you are developing something and need external services to call back to your development machine. You don't want to have to deploy each and every time to a public server. For instance, when you want to develop some webhook handlers for Shopify's webhooks.
So you have a few options how to expose your computer from within a private network. Configure your router if it allows you to and if you want to mess around with that. Limits you to the router though. What if you're sitting in a cafe or airport?
If you have a public server you can establish an SSH tunnel. Free and secure if you already use that server anyway.
Then there's a few freemium and paid services such as ngrok that come along with nice to have features like monitoring, domains etc.
