Keyword indexing means Amazon's search algorithm recognizes that a specific keyword is relevant to your product listing and will display your product when that keyword is searched. A product must be indexed for a keyword before it can rank for it.
If Amazon does not index your product for a keyword, it will never appear in search results for that term — regardless of how relevant the keyword is. Verifying keyword indexing is a critical step in listing optimization.
RIDGE keyword analysis includes indexing gap identification — keywords that should be driving traffic to your listing but may not be properly indexed due to listing optimization issues.
A seller discovers their listing is not indexed for 'yoga mat thick' despite having it in their title, because Amazon's system has not yet processed the listing update. Re-indexing by editing the listing and saving resolves the issue.