Before explaining what the reindex , we have to define first the concept of index.
In general, information such as the status of your stocks or the price of your items are stored in a database. This information is not just stacked in the database, but are stored so that they can find them quickly when needed . An index is an element of the data storage system.
On Magento , the indexing system is made in order to group all indexes , so all the features of all your products in one and only place. Whenever there is a change made to a product you must re-index the table , that is to say, update it for the modification appear on the site .