My battery was low so i jumped the car it started right up and i tested the voltage it was 14.2 after running for ten minutes the battery light came on and the the voltage just kept dropping till i shut off car at 8.4 volts what did i do please help

When i jumped the car i noticed a small white plug with a red and a blue wire i thought i might have unplugged it by accident but i cant see anywhere for it to plug int and there is a yellow plug on positive side of battery what does it do it is plugged in thanks