what is this wire?

[vmb]

I had an Optima die on me due to not driving the car and not being plugged in to the Tender. I tried to revive using a microprocessor CTEK charger, but it failed to fully charge after a week. So, I bought a new one and put it in yesterday and noticed this wire sitting there in the battery area, unplugged from its female end. What is it? Should I plug it back in?




Thanks!