Simply put, a book's primary conflict (also sometimes called the central conflict) is the problem or question that's raised toward the beginning of the book and resolved at the Resolution.
Incognolio by Michael Sussman. Problem: The nonsense word incognolio is stuck in the Author's head and won't leave. Question: Will the Author discover what incognolio is and finish his book?
Parable of the Sower by Octavia Butler. Problem: Olamina's community is in constant danger. Question: Will Olamina survive and find or create a safer, more stable community?
Labyrinth (1986 film) by Jim Henson. Problem: The Goblin King has stolen Sarah's baby brother, Toby. Question: Will Sarah solve the Labyrinth in time and get Toby back, or will Toby be turned into a goblin?
My understanding of a book's primary conflict affects the way I construct a reading of the manuscript as a whole and often provides information crucial to a) understanding why certain elements aren't yet gelling, b) proposing solutions, and c) recognizing opportunities to make the book even more impactful. How does it do this, and what kind of information do I glean from it?