The main idea might be the joy that the Christmas tree brings. It's that simple. Just the sight of a beautifully decorated Christmas tree can make people happy during the holiday season.
The main idea could be about renewal and new beginnings. The Christmas tree, being evergreen, stays green throughout the winter. In the context of the story, it could symbolize a fresh start, like the new year that is approaching. It also represents the cycle of life, as it is a living thing that is brought into our homes to be a centerpiece of celebration. And, of course, it's related to the giving spirit of Christmas. People put presents under the tree, which is another important part of the story.