During a leap year in the Jewish calendar, the extra month of Adar is introduced, and the holiday of Purim is celebrated during it. The Jewish lunar year has 11 days fewer than the 365-day solar year. If nothing were done, Jewish holidays would shift from one season to another. However, the Torah states that the holiday of liberation from Egyptian slavery (Pesach) must fall in the spring, and Sukkot in the fall. Therefore, the introduction of a leap year aligns the international solar calendar with the Jewish calendar.
Our sages decreed that if seven leap years with an extra month are introduced for every 19 lunar years, the lunar calendar will exactly match the solar calendar at the end of that period. It is accepted that in each such cycle, the 3rd, 6th, 8th, 11th, 14th, 17th, and final year, the 19th, were leap years. The accuracy of our sages' calculations can be judged by the fact that over the course of 1,500 years, not a single adjustment was necessary to the calendar they calculated.