A Battery Management System, or BMS for short, is used to protect your battery during charging and discharging.
During charging, the BMS will monitor the voltage of all of your cells and balance the cell groups to ensure they are charged equally.
During discharging, the BMS will monitor the voltage of all of the cells, as well as the entire pack voltage and the discharge current. If the BMS determines that any of its preset limits have been passed, such as the battery draining too low or an unsafe amount of current being pulled from the battery, the BMS will cut power to the battery to protect it from damage.
This 3s BMS is fairly easy to install, but will require that you are decent with a soldering iron to add the charge/discharge/balance wires (and you can get the wires here or the connectors here). You should use fairly heavy gauge wires for the charge/discharge wires, while very small wires can be used for the balance wires as they carry much less current.
The wiring diagram for this BMS is shown below, and you can read an article showing you how to install a BMS on your VRUZEND kit here. Please make sure you read the instructions in that article so you can install your BMS properly. Note: Be sure to use sufficiently thick wires for B+, B-, P+ and P- to carry the current that your device requires.
Number of cells in series: 3
Maximum rated charge/discharge current: 25 A
Charging voltage: 12.6 V
Balance current: 50 mA
Operating temperature: -4ºF to 122ºF (-20ºC to +50ºC)
Board dimensions: 1.75″ x 2.25″ x 0.2″ (4.5 cm x 5.6 cm x 0.4 cm)
Note: this BMS is designed for lithium-ion cells, and will not work for LiFePO4 cells.