You should very rarely need to trigger indexing manually. Magento is running jobs every minute to try and reindex any recent changes. This includes Admin updates for categories, products, catalog rules, inventory/stock, etc. It also includes mass updates via API, etc.

When working in or looking at data in the Admin, indexers do not affect it. The Admin does NOT make use of indexers (or cache) when showing you data. So it is important to always check that the actual data in the Admin looks correct first, because if it is not, then no amount of indexing will help.

You should also always check to determine if you are "not seeing changes" on the front-end because of cache, specifically full page cache, as it is more often than not the issue. Make sure indexers are not in the middle of running because things may or may not correct themselves once finished.

That said, Magento is not perfect and there are times when a manual triggering of indexing is needed.

• When Enabling a new product. We know that when new products are "added" to the site and enabled for the first time, Magento is not properly re-indexing inventory/stock when the product gets enabled. Products that the Magento indexer think are out of stock (meaning Admin might show in stock, but it did not get indexed like that or at all), do not show on category pages and show on PDP page with missing image and no details/swatches/add to cart button, etc.
• When product(s) is in stock and the PDP looks correct, but it is not showing in its categories.
• When everything else looks correct, can re-index to see if it fixes. If it does not, then need to dig deeper to see what might be off.

While re-indexing will not harm the site per se, it is not ideal to be doing over and over again, so it should really only be done when something is not looking right on the front-end and after Admin data has been checked and FPC flushed at least once.

• Status Column - "Reindex Required" does not mean you need to trigger a reindex, it means that it has been marked for re-index and Magento should reindex during next run.
• Pending Count Column - While doing a reset or having a status of "Reindex Required" means doing a full re-index, the vast majority of the time, Magento is just trying to do "partial" reindexing, i.e. trying to be more surgical and just re-index the things that recently changed (often that is just a few products at a time, but sometimes it can be a big number). The Pending Count show that at any given time.

If thinking a re-index is needed, please look at the Status and Pending Count columns to see if indexing is already in progress or pending. If all are set to "Ready" and all are set to 0 Pending, then nothing is in progress or pending, and you can trigger what is needed.

Some indexer automatically trigger other indexes as well, i.e. the Product Price indexer automatically triggers the Catalog Search indexer when done. The Stock and Inventory indexers atomically trigger the Product Price indexer. This normal and is because some indexer are closely related.

Actions: Invalidate

When you "Invalidate" an index, it is telling Magento that this index needs a "FULL" re-index. Magento will see this in the next indexing run (every minute assuming no indexing already running) and do the re-index.

An index can take anywhere from 10 seconds to 1-2 minutes to upwards of 10 minutes to complete. Product Price and Catalog Search take the longest. Run times change as we add patches, fix issues, do upgrades (should speed up), as well as when we add more products, categories, customer group, stores/websites (can slow down).

You can refresh the page to "watch" the progress.

The indexes that can be Reset when we think products are missing from site or look out of stock are Stock | Inventory | Product Price | Catalog Search

The indexes that can be Reset when we think product PDP looks correct, but not showing on category pages are Category Products | Product Categories

After the indexing is complete, you may or may not need to flush Full Page Cache.

Note: All indexes should always be set to Update on Schedule. However, Customer Grid is set to Update on Save on purpose (per Adobe recommendation) and should stay that way