In short: a key information document (KID) is a short document with a common layout that, under EU law, you receive before buying a packaged investment product — showing its risk, scenarios and costs.
Simply put: The product's manufacturer draws it up, and the seller or adviser gives it to you before the transaction. It is at most three sides of A4. It contains the type of product, a risk indicator on a scale from 1 to 7, possible scenarios, the costs, the recommended holding period and how to complain. A complex product carries a comprehension alert: "You are about to purchase a product that is not simple and may be difficult to understand."
Example: The KID says: risk 5 out of 7, recommended holding period five years, extra cost if you leave after one year. If you need the money in two years, the product does not fit your time frame.
Watch out: scenarios are models, not promises.
Source: Regulation (EU) No 1286/2014, Arts 6, 8, 13 — https://eur-lex.europa.eu/eli/reg/2014/1286/oj; Delegated Regulation (EU) 2017/653