A cargo drone was a starship without a crew that was used for transporting cargo.
In 2365, the USS Enterprise was assigned to escort a convoy of cargo drones. The convoy later came under attack by the Nuarans and one cargo drone was destroyed. (TNG novel: Power Hungry)
When Miles O'Brien was extremely busy with repairs to Deep Space 9 in 2369, he briefly wished he had transferred to a cargo drone, as there would be "no people, no complaints". (DS9 episode: "Babel")