Wont start..like i have NO battery

Ok, went over today since it wasnt raining. The battery is good, at about 90%, took out oand put on charger for just incase. The fuses are all good, the battery cables are all good as well, Tested all with a multimeter. Now my question is this pic. What do you call the circled part? Im tninking it might be bad, but i dont know how to test it.