Translation:I put a cup on the desk.
People are getting negative feedback some dumb reasons on this language. Guys, if you dislike someone's comment, please don't mark them down for simply asking a question. Help them, we're all here for the same reason. Shanton, to say kept rather than placed, it would be better to say おいていました, as to keep something somewhere is a continual action, rather than when you place something down. Hope that helps!
コップ is from the Portuguese copo, meaning "drinking glass". If you're strict about the definition, it's a glass rather than a cup, though sometimes people call other things コップ.
カップ can be used for various kinds of cups, including a bra cup, a measuring cup, or a drinking cup (especially in the case of a コーヒーカップ koohii kappu).
おきます (okimasu) - I put/ I will put (present/future tense)
おいています (oite imasu) - is being put/has been put (present progressive tense, but the literal translation isn't quite accurate)
おきました (okimashita) I put (past tense)
So duolingo intended this sentence to be in the past tense, but "put" in English can be present or past tense because it's spelled and pronounced the same either way, and all of these might work in their own way.