Amazon ranks listings primarily on relevance and sales performance. Relevance comes from whether your listing is indexed for a search term through title, bullets, backend keywords and A+ content. Performance comes from click-through rate, conversion rate and sales velocity for that term. A listing cannot rank for a keyword it is not indexed for, no matter how much you spend.
Indexing comes first
If a keyword appears nowhere in your listing, Amazon has no reason to show you for it. Check indexing before investing in ranking — searching the exact keyword plus your ASIN is a quick way to confirm.
Then sales velocity for that term
Amazon rewards listings that convert the searches it shows them for. Sales driven by a specific keyword strengthen your organic position for that keyword, which is the mechanism behind using PPC to support rank.
This is also why conversion rate matters twice: it lowers your ad costs and it improves the organic position that reduces your dependence on ads.
What does not work
Keyword stuffing the title degrades readability and click-through, which harms the performance half of the equation. Repeating a keyword more times does not increase relevance once you are indexed.
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