A series mod is a mod which uses two or more batteries “stacked”. Unlike in parallel mods, series mods have each battery’s positive wired to the next battery’s negative. This means the Voltage output is added together and the continuous amp rating stays the same.
Using the same example batteries as the parallel mod entry, two batteries with the following specs: 2500mAh each, charged to 3.7V, with amp ratings of 30A each. Placing these in a series mod your final output and capacity would be: 2500mAh, with 7.4V output, and 30A potential amperage output.