Abel's Inequality is a mathematical theorem that provides a useful way to estimate the sum of products of two sequences. It states that if you have two sequences, one non-increasing and the other non-negative, the sum of their products can be bounded. This is particularly useful in analysis and helps in understanding the convergence of series. The inequality is named after the mathematician Niels Henrik Abel, who made significant contributions to the field of mathematics. Abel's Inequality is often applied in various areas, including real analysis and functional analysis, to establish results about series and integrals.