Conversations about "shock" and the act of "shocking the pool" are amongst the most confusing topics for pool owners; they may be the most confusing overall topic of pool ownership, especially the topic of when to shock the pool. Additionally, there is so much debate and inconsistency about shock and shocking the pool amongst industry professionals; the debate and inconsistency is primarily about when to shock the pool. If the pool industry cannot come together with clear and concise answers, then how can you truly understand shock and the act of shocking the pool? In the industry's defense, there is no clear and concise answers for shock and shocking the pool, primarily when to shock the pool.
To begin with, there is a difference between a sanitizer of pool water and an oxidizer of pool water.
Products such as chlorine, bromine, and salt (to name a few) are sanitizers. A sanitizer is ultimately responsible for removing bacteria, pathogens, organisms, algae spores, dirt, debris, particles, and other contaminates from the water so that the water remains clean, clear, and safe for swimming. A sanitizer will also help oxidize organic matter in the water as best as it can to remove as much of that organic matter as possible; all sanitizers are also oxidizers, at least partly.
Shock is an oxidizer, as it will remove any organic matter (primarily ammonia, nitrogen, and other organic compounds) that the sanitizer could not remove on its own, which will free-up the sanitizer to once again remove all of the undesired contaminates in the water as well as assist with some initial oxidizing of some organic matter.
Ultimately, then, the shock revives the chosen sanitizer. A sanitizer - any pool water sanitizer - can only do so much for so long before it has to be revived - by shock.
There is a lot of science and chemistry required to define and discuss a sanitizer and an oxidizer as they relate to water chemistry; there is a lot more science and chemistry than you need to know concerning your pool water. Actual water chemists will suggest that we're over-simplifying the concepts of "sanitizer" and "oxidizer" and they would be correct. However, we're not writing for water chemists; we're writing for residential pool owners, who are extremely confused about shocking the pool and we're using a strategy to simplify the concept and process of shocking the pool as best we can to eliminate the mass confusion about the topic.
You can read the Pool Manual for additional information about the differences between sanitizing pool water and oxidizing pool water as well as isolated information about shock and the act shocking the pool.