The group field helps you group multiple fields.
For e.g. if you want to add a link with a label and an URL then you would want to group them together and make the whole group field a required field. That way, admin user won’t miss any of those fields.
ACF group field helps in organizing your fields as well as makes the UI a bit more simplified.
When you save a group field, it saves both child and parent field names. For e.g. if a you name a group field as ‘parent’ and a sub field as ‘child’ then it will be saved as ‘parent_child’ in the database. You can load it using the same name.
ACF Group Field Example
Here are some screenshots from ACF official docs.
This is how we can loop through those fields: