It refers to the way the cells are wired, in series or parallel. I found this on internet:
A battery is a group of cells.
A cell has a capacity, measured in Amp*Hours (pronounced "Amp Hours" ), abbreviated as "Ah".
A cell has a certain amount of potential to do work, measured in Volts, abbreviated as "V".
When you put cells in series, their voltage is added together. For example:
-----> [- 3.7V +] [- 3.7V +] [- 3.7V +] ----->
This battery has 3 cells in series, so this is called a "3S" battery. The total voltage of this battery is 3*3.7V = 11.1V.
When you put cells in parallel, their capacity is added together. For example:
.--> [- 1.2Ah +] --.
---|--> [- 1.2Ah +] --|--->
'--> [- 1.2Ah +] --'
This battery has 3 cells in parallel, so this is called a "3P" battery. The total capacitance of this battery is 3*1.2Ah = 3.6Ah.
You can use combinations of cells in series and in parallel to achieve different levels of Voltage and Capacitance. For example:
.--> [- 1.2Ah, 3.7V +] [- 1.2Ah, 3.7V +] [- 1.2Ah, 3.7V +] [- 1.2Ah, 3.7V +]--.
---|--> [- 1.2Ah, 3.7V +] [- 1.2Ah, 3.7V +] [- 1.2Ah, 3.7V +] [- 1.2Ah, 3.7V +]--|--->
'--> [- 1.2Ah, 3.7V +] [- 1.2Ah, 3.7V +] [- 1.2Ah, 3.7V +] [- 1.2Ah, 3.7V +]--'
This battery has 3 parallel groups of 4 cells in series, so this would be called a "4S3P" battery.
The total voltage of this battery is 4*3.7V = 14.8V, and the capacitance of the battery is 3*1.2Ah = 3.6Ah