Rider

Meaning
News arriving, and the speed at which it arrives. The Rider is movement toward the querent from outside — a message, a visitor, a development that has been travelling and is now close. Where the Letter is the document, the Rider is the delivery: the fact of something coming, more than its contents. It carries a sense of imminence rather than distance. In older readings it also stands for a young man, particularly one who is active, mobile, or peripheral to the situation rather than settled inside it.
The tone is bright but light. The Rider brings, it does not decide. It says something is on its way; it says very little about whether that thing is welcome.
Beside other cards
Usually the modifier rather than the subject. It makes its neighbour faster, nearer, or newly announced — Rider + Ring is news of an agreement, Rider + Coffin is news of an ending. When it follows rather than leads, it points to something moving away from the querent instead of toward them.