Magento gets a lot of data from merchant’s e.g., users, price in database tables. To optimize the store front-end you need to keep indexes up-to-date which shows the actual data on the storefront.

What Happen if NOT Re-Indexed?

If you not re-indexed after any update it will show the old data to the frontend. It will also affect the speed very badly.
Magento 2 Re-Index can be done with two methods.
1- Backend/Admin Panel
2- Command Line CLI

How to Re-Index Admin from Admin Panel?

You can re-index in two different modes from the Admin Panel.
• Update on Save
• Update on Schedule
To do so-

Go to system and under Tools, Click Index Management
Select the checkbox to change
Select Indexing Mode from Action Dropdown
Click Submit

How to Re-Index via CLI?

You can reindex all or select a specific indexer. Run the following command in CLI.

bin / magento  indexer : reindex [indexer]

To view, the List of Indexers run the following command.

bin / magento indexer : info

You can also view index status by running the following command.

bin / magento indexer : status

For more Detail Visit - https://www.fmeextensions.com/blog/magento-2-reindex-from-admin/