Every time you save a product or a category, Magento produces data re-indexing in a database. Providing that the catalog is not too large, this process is imperceptible for a user. However, the more categories, products, and attributes of a product are created, the more time for re-indexing is needed. As a result, working with the administrative panel becomes uncomfortable being that you have a long wait long until the category or product will be saved after editing.
The graph below shows the increase of time for saving, depending on the growth of the size of the catalog.
(category) or changes an existing product, the query to produce re-indexing of goods is stored in the table-queue. Products are quickly stored and the manager goes to editing the next item.
Parallel to this the cron process runs constantly addressing the table-queue. As soon as it notes the elements that are expected for re-indexing, it starts to index them.
You may have a very large catalog and a large number of managers simultaneously working with it, but they will not wait until re-indexing happens every time they save data. They will continue to work, and the cron process will perform data re-indexing in the background.