Basically its a way to share files or access to an application or server without needing to mess with any networking stuff (like port forwarding, buying and setting up a human readable domain name, etc.).

For example: You can spin up a website on a raspberry pi and use zrok to generate a url for it. Now anyone with the url can access it like a normal website. You can apparently also make the website private so that only those with the url, zrok, and the private share token can access it.

Another example: Zrok can also generate a download link to a file you own. The link can be public or private as well.