App Description is the long-form text on a store listing — on Google Play it directly affects keyword ranking, while on the App Store it is conversion-only and not indexed for search.
The description's role differs sharply by store, and confusing the two is a costly mistake. On Google Play, the algorithm reads the long description for keywords, so natural keyword placement and density matter for ranking — terms used a few times across the description help you rank for them. On the App Store, the description is not indexed for search at all; its only job is to convince the shopper, so you optimize it purely for persuasion and clarity.
On both stores, most users only read the first two or three lines before the "more" fold, so the opening must carry the value proposition. Below the fold, scannable structure — short paragraphs, feature lists, social proof — does the convincing work for the minority who keep reading.
Example
A photo editor mentions "background remover" three times naturally across its Google Play description to rank for it — but on the App Store it relies on the keyword field for that term and uses the description purely to sell the feature. On the App Store, spend the description budget entirely on persuasion; the keyword field does the ranking work.