An "Editor Group" is a group of settings and features for the WYSIWYG visual editor that can be assigned to any user.
Each user can be assigned to a different Editor Group, so (for example) it is easy to allow some users to have ALL toolbar options while restricting other users to only a few options. You can also specify general editor options here. For example, you might want certain users to not have access to the wysiwyg editor, or you might want a default width size of 100%.
NOTE: Any "editor specific" Snippet Tag attributes will always override the Editor Group settings assigned to the user. For example, if you specify the wysiwyg="yes" and width="100%" Snippet Tag attributes, then the wysiwyg editor will always be used with a width of 100% when editing that snippet.