Re-index feature is meant to increase the performance of Product Preview:
This feature indexes the main attributes used in Product Preview:
Product Preview attributes - Name, EAN, SKU, Price, QTY.
After re-mapping these attributes you will get a notification to re-index your suppliers.
You can hover over the icon near the supplier name to get additional info
Then press “Reindex supplier” button on the top right corner
After pressing it you will be asked “Do you want to reindex suppliers?”. Press “Yes” if you are done with attribute mapping and suppliers will be added to queue for re-index.
During this process you will not be able to use product Preview search for suppliers in re-index queue. Re-index is done when all warning icons and statuses are gone and you can use Product Preview without restrictions.