Keyword Field is the App Store's hidden 100-character metadata field where you list comma-separated keywords that count toward search ranking but are never shown to users.
The keyword field is unique to the App Store (Google Play has no equivalent — it reads keywords from the visible description instead). You get 100 characters per localization, and the App Store indexes them for search but never displays them. The optimization game is fitting the maximum number of rankable terms into those 100 characters: separate words with commas and no spaces, never repeat a word that's already in your title or subtitle, drop filler words like "app" and "the," and rely on the algorithm to combine your single words into multi-word phrases.
Because the field is invisible, it's pure ranking inventory with no conversion cost — every wasted character is a keyword you could have ranked for. Across all the localizations a storefront indexes, the effective keyword space multiplies, which is why keyword-field discipline and cross-localization go hand in hand.
Example
Instead of "best budget app, money saving app" (wasteful), you write "budget,money,saving,expense,finance,tracker" — the algorithm recombines those into phrases like "budget tracker" and "expense finance," fitting far more coverage into 100 characters. Skip plurals the algorithm already infers, and never waste characters on words sitting in your title or subtitle.