This is a broad question and can be answered in many ways. From a more traditional and religious point of view, Christmas is about the celebration of the birth of Christ. To be sure, from a historical point of view, no one knows the date of the birth of Christ, but the church celebrates it on that day - December 25th. So, in light of this, for many people around the world, Christmas is preeminently a religious holiday that remembers the birth of a savior. The gospels of Matthew and Luke have the birth narratives of Christ.
For others, Christmas is more of a secular holiday, where people wish each other well and even give gifts to those they love. For these people it is a day of good will towards others.