A downloadable Amazon PPC report showing the actual customer search terms and ASINs that triggered your ads, along with their performance (clicks, spend, sales, ACoS) at the search-term level.
Two uses: pruning and promotion. Pull it regularly to find irrelevant terms to add as Amazon Negative Keywords / Negative Product/Brand Targeting, and to find well-performing discovered terms or ASINs to promote into their own dedicated exact-match campaign — see 'Good Keywords' Scaling Campaign. It's the primary data source that makes an Automatic Campaign Four-Way Split (Close Match, Loose Match, Substitutes, Complements) campaign useful as a discovery engine rather than an end-performance channel.
A search term report becomes available within 24 hours of a campaign going live. Clearly irrelevant search terms should be added as negative keywords and irrelevant ASINs as negative product targets as soon as they're identified. However, a high ACOS (50-100%) in the first few days should NOT by itself trigger negation — the data sample at that point is too small to be reliable. See PPC Honeymoon Period.
Check regularly early in a launch, especially on auto campaigns, to catch two things: irrelevant search terms to negate (Amazon Negative Keywords) and relevant, unexpected terms to fold into exact-match campaigns (Search Term Harvesting into Exact Match) and into listing copy.