The sprint points per day is used to convert point based estimates into days and day based estimates into points. If your work estimates are only entered / viewed in days you can ignore this value.
If the resource is a named resource, enter the points you expect them to complete each day. If the resource is a group, enter the points you expect an average resource in the group to complete each day.
The value entered in the edit sprint window is a default and can be set specifically for each work item.