Product structured data
Product structured data is machine-readable markup embedded in a product page, usually a schema.org Product object in a JSON-LD block, stating the name, price, availability, identifiers and variants as data rather than as layout.
The value is that it removes guessing. Without it, a client has to work out which number on the page is the price, and pages are full of numbers: a crossed out price, a per month figure, a bundle total, a delivery threshold. With it, the price is the field called price.
Getting it partly right is common and worth checking for. Markup that omits availability, or that describes only the default variant, answers half the question and leaves the agent to improvise the rest.
Why it matters to a store
This is the highest ratio of benefit to effort on the whole list. It is usually a theme level change, it helps ordinary search at the same time, and it is what every protocol reads from.