The amount both sides of a market add up to above 100% — the book's cut, and the number every edge calculation has to be measured against.
TODO — write this one yourself.
TODO — cover how hold compounds across bet volume, why comparing to the priced number instead of the fair number invents edges that do not exist, and where the even-split method breaks down.
Math, not advice. 21+ where legal. If gambling stops being entertainment, call 1-800-GAMBLER.