With the Merge/Center option you actually merge two or more cells into a
single large cell with your data in the center of it. With Center Across
Selection the selected cells all remain separate with the data merely
displayed in the center.
By the way, most Excel experts recommend avoiding merged cells as far as
possible as they can cause problems with calculations, filtering, sorting etc.
if you merge and center it merges and centers in cells that datas in, if you
merge and center across selection you can select how many cells you want
merged.