Two players rattle out eight moves in thirty seconds and then one of them sinks into a twenty-minute think. Nothing dramatic happened. They ran out of book.
8 
7 
2 
a b c 
d 
e h 1 Theory is that settled knowledge. Mostly openings, though the word also covers endings that have been solved (rook and pawn against rook is theory too). A book move is a move that already appears in it. The book itself used to be an actual book; now it is a database of millions of games, and the phrase survived.
What is in there
Move orders, mainly, and the reasons behind them. The Ruy Lopez above has been played since the sixteenth century, when a Spanish priest called Ruy López de Segura wrote about it, and every sensible third move for Black has been tried a hundred thousand times. That accumulated experience says 3…a6 is good, 6.Re1 is the main move, and White’s plan involves c3 and d4.
None of it is proof. Theory changes, engines overturn assessments that stood for decades, and a line that was dead in 1990 can be respectable now.
Where it stops helping
At the moment your opponent plays something not in it, which at club level is usually move five or six.
That is the moment the memorising was never going to help you anyway. What helps is the same handful of ideas the theory is built from: put a piece on a square where it does something, take a centre square, get the king castled. Those are on the development page, and they are what you fall back on every single game.
How much to learn
Less than you think, and in this order: the first four moves of one opening as White, the same as Black against 1.e4 and 1.d4, and the reason for each move rather than the move itself. A player who knows why 3.Bb5 pressures the knight that defends e5 will find the right idea on move seven; a player who memorised twelve moves will not.
When your opponent leaves the book with something you have not seen, he has not necessarily found an improvement. He may just have played a bad move first. That distinction, and the word for a genuinely new move in a known position, is on the novelty page. And for the openings themselves, the opening puzzles will teach you more about what actually happens after move eight than any repertoire file.