What does trimming the Christmas tree mean?

I believe it means "to Decorate the Christmas Tree."

Trimming the tree comes from the days when everyone harvested their own tree. No tree was ever the perfectly cultured trees we have today, nor were they selected from "farms" by professionals, they were collected by each family as part of their celebrations. The "imperfections" of the tree (such as odd, or extra long branches that may tip the tree) were trimmed off to make the tree more stable and pretty. This was usually done just before the lights, and ornaments went up, thus we still talk of trimming the tree as making it look pretty.