Brumation is similar to when mammals hibernate for the winter. Metabolism slows down, the dragon will stop basking and eating (mostly) and will sleep for a few weeks/couple of months.
Some dragons only partialy brumate, they stay awake and bask for a couple hours then go to bed early, they generally don't eat during this time. Not all dragon brumate.
Normally it doesn't start until a dragon is a adult or older juvie. Your baby won't brumate, he is too young this season. Watch for it next winter.