Definition
Placing on the market is the first time a specific product is made available on the EU market, whether it is sold, distributed or handed over for use. The word "first" matters. It is the initial supply of that individual unit, not every later resale.
This moment is the trigger for compliance. The product must already meet the applicable requirements when it is placed, so the CE mark, the declaration of conformity and the substance checks under RoHS and REACH all have to be in place by then, not arranged afterwards.
It also freezes the legal baseline. A product placed on the market must satisfy the rules as they stood at that date, so a later change in the law does not retroactively make a compliant unit non-compliant. Which of the economic operators does the placing depends on the supply chain, with the manufacturer or importer usually carrying the heaviest duties.
Note: general educational information, not legal advice. Check the official source before relying on it.