Usually when a book is part of a series, we try to sort out the significance of the titles for both the series and the particular installment. But in the case of The Dark is Rising, the series is named after the book. So consider this a twofer. As for what the titles means, check out what Will finds himself chanting about the Dark:
"For the Dark, the Dark is rising. The Walker is abroad, the Rider is riding; they have woken, the Dark is rising. And the last of the Circle is come to claim his own, and the circles must now all be joined. The white horse must go to the Hunter, and the river take the valley; there must be fire on the mountain, fire under the stone, fire over the sea. Fire to burn away the Dark, for the Dark, the Dark is rising!" (3.60)
The Dark, it seems, is taking over the world—evil is coming—and it's up to the Old Ones to stop it. The title hints at what's to come if the Old Ones don't take up their quests and defeat the Dark; it will keep rising. Gulp.