Metadata is the collection of text fields — app name, subtitle, keyword field, description and more — that tell a store what your app is and which searches it should appear for.
Metadata is everything you type into your store listing, and the app stores read two kinds of meaning from it. The indexable fields — app name, subtitle, and the hidden keyword field on iOS; the title, short description, and long description on Google Play — feed the search algorithm directly and decide which queries you can rank for. The display fields, like the promotional text and the first lines of your description, mostly do conversion work once a user is already on the page. Strong ASO treats these as different jobs done by the same text.
Because most metadata fields are capped and weighted differently, the craft is allocation: place your highest-value keywords where the algorithm rewards them most, never repeat a term across fields where one mention already counts, and keep the copy readable for the human who has to choose. Metadata is the one ranking input you fully control, which is why it's where almost every ASO project starts.
Example
An app might index for "budget planner" through its subtitle and keyword field, then use its promotional text — which isn't indexed — purely to announce a seasonal sale and lift conversion.