Addition is a regrouping: two sets of objects which were counted separately are put into a single group and counted together: the count of the new group is the "sum" of the separate counts of the two original groups.
Addition is a regrouping. Two sets of objects are put into a single group. These objects were counted separately. Two sets of objects are counted together. These objects were counted separately. The count of the new group is the "sum" of the separate counts of the two original groups.