Runs allowed per nine innings pitched -- the title says it all. It's basically ERA with the "E" removed.
For example, in 1972, Nolan Ryan allowed 80 runs in 284 innings, giving him a 2.54 RA9.
Runs/IP x 9
RA9 is simply a cousin of ERA, and the player rankings for each are more or less the same, with a few minor differences.