A battery management system, or BMS, is the control electronics inside a lithium battery pack. Its job is to monitor and protect the cells.
Lithium battery packs are made from multiple cells connected together. Those cells need to stay within safe voltage, temperature and current limits. If one cell drifts too far from the others, the whole pack can lose capacity or be damaged.
The BMS watches cell voltage, pack temperature and current flow. If charging goes too high, discharge goes too low, or temperature moves outside safe limits, the BMS can limit or disconnect the load.
A good BMS also helps balance cells so they age more evenly. Balanced cells improve usable capacity and reduce the chance of one weak cell limiting the whole pack.
For vehicle-mounted power systems, the BMS is not a bonus feature. It is part of the safety architecture that allows lithium batteries to work reliably around heat, vibration and changing loads.