Stolen goods is a term for a cargo canister collected by someone other than its owner. Most canisters that can be collected using the ship's Cargo Scoop in space are considered stolen.
Note - If you have not personally purchased the goods in your cargo hold from a market then it will be seen as stolen property because it initially belonged to someone else unless the original owner jettisoned it using the "Jettison Cargo (Abandon)" command from the cargo screen.
The status of cargo another ship is carrying can be verified by using a Manifest Scanner on a targeted ship and then viewing the CARGO menu in the left hand screen of your own ship. If any ship is cargo scanned while carrying stolen goods, the pilot will incur a bounty! Stolen goods may be sold at any station offering a Black Market in the Contacts menu of Starport Services.
Carrying stolen goods can be a dangerous pastime. Authorities, and other players, will often scan a ship's cargo to see if they are carrying any stolen goods.