B+ is "battery positive" or 12-14volts. Basically, it's saying "are you getting the same voltage at the fuse as you do on the battery's positive terminal". Generally, a reading of 1 means you're reading above the range of your current setting. Like set for a 2 volt range and trying to display 12 volts...it will just show a 1. Make sure you're on the 20 volt range, DC.