Battery charge Indicator. Directly connect this circuit board to you battery , positive to battery + and negative to battery - . 
It will show battery charge left. If 4 led glows then its 100% charge, if 3 led then 75%, if 2 led then 50%, if 1 led then 25%. And if no led then battery drain out and need to charge again.
This module is for only 7.4v 2S lithium Ion or lithium Polymer Battery. not for lead acid Battery.