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 4s BMS is fairly easy to install, but will require that you are decent with a soldering iron to add the charge/discharge wires (and you can get the wires here or the connectors here). 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.
Number of cells in series: 4
Maximum rated charge/discharge current: 30 A
Charging voltage: 16.8 V
Balance current: 60 mA
Operating temperature: -4ºF to 140ºF (-20ºC to +60ºC)
Board dimensions: 1.75″ x 2.25″ (4.5 cm x 5.6 cm)
Note: this BMS is designed for lithium-ion cells, and will not work for LiFePO4 cells.