Complex sentence: 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.
Split into simpler sentences: Addition is a regrouping: two sets of objects 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.