The subscriptions API can be used to notify a smartapp that events have occurred. In the documentation here it says:
Subscriptions API Basics
Subscriptions belong to a specific installed instance of a SmartApp. The Subscriptions API follows the path pattern of https://api.smartthings.com/installedapps/{installedAppId}/subscriptions.
Subscriptions are typically created or updated during the INSTALL and UPDATE lifecycles, when the user has selected devices and granted the requested access to your SmartApp.
When a subscribed-to event occurs, your SmartApp will receive a POST request containing information about the event, so your app can react accordingly. See Subscription Event Handling for more information.
So, you would need to host a smartapp of some sort that would receive a POST when an event occurs.
The API is a websocket API. It is a little different than a SmartApp, but very similar JSON. At the time of that post I was with a large company developing a homeautomation/entertainment application on Windows, integrating SmartThings. We had to register as a Partner I believe, and received from Samsung/SmartThings a document and the JSON protocol, to send changes, and receive changes to the ecosystem via the Websocket.
I am not sure if SmartThings supports this API any longer. But …
If it does, then it would depend upon your applications expected network topology as to whether you use the API described @h0ckeysk8er (SmartApp) or this WebSocket (No Server, SmartThings App) api.
I wish I had the link for you to sign up, I recall doing that, but can’t seem to find it.
The only freely documented way of subscribing to events is to create a SmartApp, as has has been mentioned.
The Samsung Automation Studio, which is based on Node-RED, can subscribe to events without having to be plumbed in as a SmartApp (though you can do that). My understanding is that it uses Server-Sent Events (SSE) for this. As Samsung make their whatever-it-is-called freely available to other Node-RED users I have rather been presuming it works for them too, but I don’t know for sure. So SSE is certainly floating around, it just isn’t documented and access to it may be restricted somehow.