In writing, b. is a short form of born. You see it in dictionaries, reference notes, museum labels, and lists of people: a name, then b. and a year. It tells you when someone came into the world. People almost never say the letter “b” out loud for this; they read it as “born.” It often sits next to d. for died, and sometimes c. for “about.” So “Jane Doe, b. 1902, d. 1988” is a tiny life story in punctuation. This sense is only that written use. It is not the school grade B, not the name of a chemical, and not the verb be.

b.（書面縮寫，＝born「生於」）