Cesium ion is the platform where you can upload, tile and host 3D data. You can do this either through the browser interface, by clicking and dragging to upload your data and selecting the tiling options, or you can do this programmatically using the REST API, either to automate it or to integrate it as part of your application.
Does that make sense? As an example, this is how the 3D Tiles FME plugin works (https://www.safe.com/integrate/cesium-3d-tiles/). Instead of having to manually take your data and upload it to Cesium ion, FME can use the REST API to upload it to your account automatically at the click of a button.