An animation group is a collection of SVG/HTML elements. Think of groups as components on the web (such as an intro or animated icon). Each group has its own controllable timeline.
Give your group a descriptive name. You can access the group later on using the Web Player:
You can either point and click in the browser or use the Chrome Devtools - Elements Panel.
Limitation is that you won't be able to select parent elements.
Using the Elements tab to add elements to Spirit Studio gives you more control.
Make sure to read how the Web Player resolve elements: