Possible meanings are 1) she had been a widow for 84 years or 2) she was a widow and was now 84 years old. (See: [[rc://en/ta/man/translate/translate-numbers]])
This is probably an exaggeration meaning that she spent so much time in the temple that it seemed as though she never left it. Alternate translation: "was always at the temple" or "was often at the temple" (See: [[rc://en/ta/man/translate/figs-hyperbole]])