An eCommerce site needs to store a lot of data, on customers, products, attributes, stock levels and more. In order to run quickly, Magento stores this data in a special format, known as an index. Without this, your site would run really slowly.
However, every time you make a significant change on your site, your data needs to be reindexed. Normally, this is a difficult task in Magento, but with the help of an extension, we can make the process much easier.
This video introduces you to how indexes work in Magento 2. https://www.youtube.com/watch?v=x1U7LHs_X-s