Chalking the door is a special tradition that some people do at the beginning of the year, especially during the season of Epiphany. It's kind of like drawing a picture on your front door to remind you of something important.
The picture usually has three letters – C, M, and B – in between the numbers of the current year. These letters stand for the wise men who visited baby Jesus – Caspar, Melchior, and Balthasar. They also stand for a special blessing, which is: "Christ bless this house."
Some people also draw a cross or other special symbols on their door to remind them of God's love and protection. This way, whenever people come into their house, they can see the special picture and remember the blessings that have been given to them.
But remember, it's important to always ask your parents or guardians before drawing on your door or anything else.