Some of it is explained here:
GitHub - EsotericSoftware/spine-runtimes: 2D skeletal animation runtimes for Spine.
There are two parts, Spine (the editor) and the Spine Runtimes. The Spine license is here:
Spine Editor License Agreement
The Spine Runtimes license is here:
https://github.com/EsotericSoftware/spine-runtimes/blob/master/LICENSE
To use Spine you need to license Spine. Like Photoshop and other tools, anything you create is your IP, you own it and can sell it, give it away for free, etc. This covers the Spine project file and export files such as JSON, binary, images, atlases, and video.
The Spine Runtimes are code that goes in your apps to load and render JSON or binary data exported from Spine. If you want to use the Spine Runtimes in your applications, you are granted rights to do so simply by licensing Spine. Note this means you need a valid Spine license to distribute your apps, so a Spine Enterprise license will need to be renewed if you continue to distribute your apps. Spine Essential and Professional do not expire except if you have more than $500k annual revenue, at which point you need Enterprise.
EDIT: Spine's licensing has been revised to allow continuing to use the Spine Runtimes without renewing your Spine Enterprise license. Please see here:
Blog: Our new licensing explained