What does S (Series) and P (Parallel) mean in a battery pack?

Often in battery packs, "Series" and "Parallel" refer to different ways of connecting individual battery cells to increase the overall voltage or capacity of the pack.

Connecting cells in series means connecting the positive terminal of one cell to the negative terminal of the next cell. This increases the voltage of the pack while keeping the capacity the same. For example, if you connect two 3.7V cells in series, the voltage of the pack will be 7.4V (3.7 + 3.7).

Connecting cells in parallel means connecting the positive terminals of multiple cells together and the negative terminals together. This increases the capacity of the pack while keeping the voltage the same. For example, if you connect two 2000mAh cells in parallel, the capacity of the pack will be 4000mAh (2000 + 2000).

Battery packs can also be a combination of both Series and Parallel connections. For example, if you connect Four 3.7V cells in series, and then connect two of those Series strings in Parallel, you will end up with a 14.8V pack with twice the capacity of a single 3.7V cell.