From the Baseball Prospectus website:
BP wrote:Value Over Replacement Player. The number of runs contributed beyond what a replacement-level player at the same position would contribute if given the same percentage of team plate appearances. VORP scores do not consider the quality of a player's defense.
It's basically how much a player A contributes above what an average bench, or AAAA player as it's referred to sometimes, player would contribute. The values can most certainly be negative, which means that a given player actually contributes less
to his team than an average bench player would, though that is sometimes misleading because, as the definition says, it doesn't take into account defense. Though that's usually only an issue with catchers.
RC/27 is the runs-created (RC) of a particular player divided by 17 (number of outs in a game). It's (roughly) how many runs a player creates for his team over the course of a game.