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The average tube set has two or more (lets ignore cases of more for now) power sources. The A supply is the heater voltage for the signal tubes, and the B supply is the supply voltage for the plates etc. (what the set uses to allow the tubes or transistors to process the signals). The B- terminal is often tied to chassis ground so when you measure the B+ you are measuring the B supply usually with referance to the chassis.
The letters harken from the 20's when all radios were powered by batteries. Some sets had a C- supply for the grids, but we got smart and found ways to make the B supply run the grids too. Sets with multiple heater supplies were not given any special names for the branches of the A supply, and sets that used resistive voltage dividers for multiple B+ voltages simply called the different B+ lines by their voltage.
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