What an annuity calculator is valuing
An annuity is a stream of equal payments made at regular intervals. The two most common questions are what that stream is worth today as a lump sum and what it could accumulate to by the end of the term if payments continue and compound.
That is why this calculator offers both present-value and future-value modes. One answers the lump-sum-equivalent question, while the other answers the accumulation question from the same payment stream.