Liber Abaci is a mathematical book written by the Italian mathematician Fibonacci in 1202. It introduced the Hindu-Arabic numeral system to Europe, replacing the Roman numeral system. The book also presented various mathematical concepts, including the famous Fibonacci sequence, which describes a series of numbers where each number is the sum of the two preceding ones. In addition to number theory, Liber Abaci covers practical applications of mathematics, such as calculations related to trade, currency, and measurements. Its influence helped lay the groundwork for modern arithmetic and contributed significantly to the development of mathematics in Europe.