Chess theory has grown enormously with the advent of powerful computers. Modern opening theory in popular lines can extend 25 or more moves deep, with both sides following precisely analyzed sequences. Top players spend significant time studying and memorizing theoretical lines as part of their preparation.
The depth of theory varies greatly between openings. Some lines like the Najdorf Sicilian or the Berlin Defense have been analyzed to extreme depth, while other openings have less extensive theory. Players at all levels must balance the need to know theory with the importance of understanding positions and plans.
For improving players, the most productive approach to theory is to study the ideas behind the moves rather than memorize long sequences. Understanding why certain moves are played in an opening teaches transferable skills, while pure memorization provides limited benefit once the opponent deviates from the book. However, knowing critical theoretical positions and their evaluations is essential for tournament play.