"x" indicates the number of cells in series.
"y" indicates the number of these “strings” that are in parallel
The voltage is additive in series (i.e. two 3.7 volt cells in series produces a 7.4 volt string); capacity (mah) remains the same as a single cell.
The capacity is additive in parallel (i.e. two 2100 mah strings in parallel will produce a 4200 mah pack); voltage will be the same as a single string.