Admin Field Group
Admin Field Groups allow administrators to group independent fields together in one location on the content entry form. This allows administrators to cluster fields together in the interface if they have something in common, while keeping them separate on the back-end. Admin Field Groups do not impact the front-end of the store at all.
A Field Collection may be a better fit for your project. Consider using a Field Collection instead of an Admin Field Group if your data points do not need to be stored separately on the back-end.
Field Settings Form
On the Field Settings page, choose from open or closed by default. This setting controls if the fields in the group are all visible or closed and hidden when the content entry form is first loaded.
Adding Fields to the Group & Sorting Fields
To put the fields into a group, drag the field handles to the right. Move the handles up or down to sort,
Content Entry Form
Below is an example of how the form looks during content entry with two fields grouped together and the group open.