It means "B" in military/aviation phonetic alphabets.
Over a noisy radio link, many letters are hard to distinguish: "bee" (B), "see" (C), "dee" (D), "vee" (V), etc. The phonetic alphabet uses distinct words for each letter in order to avoid errors.
There are multiple nitrogen tanks on the spacecraft, designated A, B... and in this case Capcom is asking for a check on the "B" tank.
You'll see frequent use of "Alpha", "Bravo", "Charlie", and "Delta" throughout the Apollo mission transcripts.
Oddly, Aldrin substitutes "Cocoa" for "Charlie" at one point:
71:14:49 Aldrin: Roger. And our readouts on board are Alpha is 82, Bravo is 84, Cocoa is 84, and Delta is 87.
Cocoa (or Coco) was used in a 1947 Latin American standard; it's possible that Aldrin picked it up during his basic flight training in Florida in 1951-1952.