🔋 2S / 3S / 4S
How many LiPo cells are wired in series. Each cell is 3.7V nominal, so 2S = 7.4V, 3S = 11.1V, 4S = 14.8V.
More cells = more voltage = more speed (and more stress on the drivetrain).
What it actually does
Cell count — the "S" number — is how many lithium cells are wired in series, and it sets the pack's voltage. 2S is two cells, 3S is three, and so on. Voltage is what determines motor RPM, so cell count is the single biggest decision about how fast your model will be and how hard it will work its electronics.
In practice
Each lithium cell is nominally 3.7V and 4.2V fully charged, so a 3S pack is 11.1V nominal and 12.6V full. Going from 2S to 3S is roughly a 50% increase in voltage, which means a large jump in RPM, current draw and heat — not a small step.
Worth knowing
Never exceed the cell count your ESC and motor are rated for, and gear down when you increase it. Over-volting is one of the most common ways people destroy an ESC on the first run of an "upgrade".