The Hebrew calendar allows us to track the Jewish year. Without that, we would not be able to celebrate the Jewish holidays at their correct time.
Unlike the other calendars in existence today, the Jewish calendar goes according to the moon. Once a month at Rosh Chodesh as the new moon appears, we restart counting the days in the month. The decision when to begin the new month used to be based upon the testimony of witnesses verifying they had seen the new moon in the sky. The Beis Din would light fires on mountains informing the people of the new moon. After a while the Sadducees began to light fake fires on mountains to confuse people and stop the information flowing from city to city. That is when they created a fixed calendar that we still use today.
An inherent challenge appears when following a lunar calendar, that the months fall out of sync with the solar cycle. Since there is roughly an eleven day difference between the eleven lunar months and the solar year, the lunar months will travel through the year and appear at different seasons every year. The Torah tells us to observe Pesach in "chodesh ha'aviv" the springtime, which is the season the Jewish people left Egypt. In order to keep Pesach in that season, every two to three years an extra month is added to the calendar, adar. By doing this, the months align themselves perfectly into the correct seasons, and thereby all the Jewish holidays too.
The only question left to answer is which month to put Purim in, the first adar or the second one? Well since we know Purim must fall thirty days before Pesach, it is celebrated during the second adar month. During the first adar we have whats called Purim Katan, (little Purim), and that falls on the same date Purim would have fallen on had it not been a leap year. We do not celebrate Purim Katan like we celebrate Purim proper, however the Shulchan Aruch recommends a feeling of festivity on that day and quotes Proverbs (15:15): "One who is of good heart is festive always".
Tonight, the 30th of January begins Rosh Chodesh Adar 1 so you have 15 days to prepare for Purim Katan and extra time to prepare for the full Purim celebration on March 16th, check here for 40 kinds of hamantashen, tons of Mishloach Manot ideas, and lots of Purim Seudah Menus.