The protection circuit board is a protection feature built into most lithium based rechargeable battery cells aimed to prevent the battery/cell from overcharging, over discharging and over draining itself. This feature is also known as the protection circuit module or PCM. PCS is a mandatory security feature required to be installed into the building of battery packs to prevent incidents of fire, explosion, and leakage. PCS is mostly brought into practice for low voltage lithium-based battery cells.
Protection circuit board functions mostly like a pressure valve which is designed to render the cell in the battery inoperative in instances of high pressure which in most cases means that the battery is charging in low voltage, in high voltage, low or high temperature or even undercharging or overcharging and similar cases. In simple layman terms, it simply works by interrupting the connection of the positive terminal from the rest of the cell. It simply disables the positive terminal from functioning any further rendering the cell inoperative.
Protection circuit board comes under the broader security concept known as a battery management system which is a sort of management system to supervise the state of your lithium bases battery pack. The system is designed to intently observe the functioning of the battery cell through intimate scrutiny of its state of function, the stability of its cells, protection of the battery and governing the environmental factors which may cause any effect.