what is satellite in adobe launch

Satellite is basically a JavaScript object that is used by Adobe. It has some functions such the allow you to retrieve or set values, some extension leverage the Satellite Object to create debugging logs, etc.

For the most part, Adobe Launch will handle most of the functionality for you... Data Elements are stored as values in the Satellite object.

If you need to read a data element in some custom Javascript you can just use the getVar function to retrieve it:

var myValue = _satellite.getVar('data element name');

You could also in theory set a value into Satellite with setVar if you really have a use case for this.

Here is more information on Satellite Objects: 

https://experienceleague.adobe.com/docs/experience-platform/tags/client-side/satellite-object.html?lang=en