What it means
A Raja yoga forms when the lord of a kendra (1, 4, 7, 10) and the lord of a trikona (1, 5, 9) come together — by conjunction, by mutual aspect, or by exchanging signs. The logic is that the pillars of a chart and its fortunate trines are pulling in the same direction.
How it is read
The name translates as 'royal combination', and that is where the trouble starts. Twelve ascendants each have several kendra and trikona lords, so some form of Raja yoga appears in a large share of charts; a few ascendants get one almost automatically because a single planet rules both a kendra and a trikona.
What to watch for
Whether it delivers is a separate question from whether it exists. The planets involved have to be strong, undamaged, and their dasha periods have to actually run during the life. A Raja yoga formed by two debilitated planets whose periods arrive at eighty is a real yoga and a thin promise.