Purim is not one of the holidays that is commanded to be observed in the Torah.
But, it is an important holiday for the Jews.
It celebrates the events portrayed in the Biblical book of Esther.
Specifically, as indicated on the Jews For Jesus Website:
“The Jewish holiday of Purim commemorates Esther's and Mordecai's defeat of Haman's plot to annihilate the Jews of ancient Persia. Purim derives its name from the fact that Haman cast lots (purim) to determine the most propitious date for implementing his planned genocide. The holiday is celebrated on the 14th day of Adar in the Jewish calendar.”
For more information, as detailed in Wikipedia, HERE’S THE LINK.