The headline says: US Air Force C-17 Makes Mysterious Visit To Moscow.
I won't bore you with details but it's this kind of a plane:

So you tell me which political mission requires a cargo plane like this one.
Kremlin confirmed that it indeed landed in Vnukovo, Moscow.
It is unlikely that the US and Russia are trading weapons: either side is neither buying, nor selling to the other side. Although, who knows.
This kind of a plane would be appropriate for transporting sizeable quantities of gold. Gold is heavy. Governments like gold, for sovereignity purposes. And there are a few things Russia may need and be able to buy with gold.
This is just speculation on my part, really, because I don't have eyes on the inside of the plane. Please don't sue.
Anyways, this thing mostly flies under the radar.