It will give a server (website or api) you have running on your local machine a public url.

Like @beaconstudio below mentioned it could be used to test webhooks. Other uses are testing local sites on mobile devices, like I just did to give the mobile experience of expose.sh a revamp. Its faster than running adb, rooting your phone or connecting a cable and doing mobile debugging.

There are other tools, although expose.sh is focused purely on http/https and not other protocols, this makes the syntax a bit easier as you don't have to specify a protocol, just a port i.e. "expose 80" instead of "ngrok http 80" (using ngrok as an example).

I've used similar services for Web hooks - if you're developing eg. a slack bot you'd have to deploy to a public url constantly without a tool like this, which shows down development considerably.

Looks like a competitor to ngrok.