In pagan tradition, the year is divided between two kings. The Oak King rules the summer, but as the days shorten after the Autumn Equinox, the Holly King takes his throne.
Holly remains evergreen when all else is bare, a symbol of life amidst the death of winter. The bright red berries were seen as drops of the sun's blood, preserved to ensure its return in spring.
Bringing holly indoors brings luck to the household—specifically to the men (while Ivy brings luck to the women). However, it is bad luck to cut down a whole holly tree; it must only be pruned.