What is Advent?
Generically, advent means "arrival, emergence, coming."
Religiously, it refers to the 4 weeks leading up to Christmas, when Christians prepare for the coming of Jesus, the light that comes to give hope during the literal darkest season of the year (coinciding with the winter solstice).
When you were little, your parents may have given you an advent calendar to help you count down the days until Christmas.
Simply, the season of Advent is to help us remember why we celebrate Christmas. If you're not religious, it's a way to remember that there's always a light to light your darkest hours.