That's a pretty popular voltmeter here on BLF. First pin is (-), 2nd pin is (+), it can connect & measure up to 6 batteries in series.
Here's some BLF pictures of that LiPo 1s-6s Voltmeter installation:
on the xtar MP1: https://budgetlightforum.com/t/-/7566
and on the ML-102: https://budgetlightforum.com/t/-/5219
Steve_the_chief's detailed pictures of voltmeter connection on MP1: https://budgetlightforum.com/t/-/5973