If there is present a relationship (combination, bidirectional vision or exchange of zodiacs) between two or more than two planets (out of which atleast one should be the lord of 'Kendra' & the other should be the lord of 'Trikon') in a birth chart than these planets are said to be constititing a 'Rajyog' in Kundli or birth chart. This 'Raja Yoga' in Vedic Astrology is said to give benefits to the person concerned related to the houses in which this 'Raj yog' has taken place or related to the houses of which the planets constituting 'Rajyog' are having a lordship. Rajyog in Kundli doesn't necessarily mean becoming a king. Usually its benefits are observed in a person's life when there is mahadasha of one planet and the antardasha of another planet in the person's life, provided both planets are constituting Rajyog in some way in the person's birth chart.
Examples of Rajyog -
Example#1: - In the chart given below there is a combination relation between Jupiter & Venus in the first house which is constituting a Rajyog because Jupiter is the lord of 5th house (Trikon) & Venus is the lord of 10th house (Kendra). Mars is itself constituting a Rajyog (without the aid of any other planet) because it is the lord planet of 9th house (Kendra) as well as the lord of 4th house (Trikon). This way there are two Rajyogs in this chart.
Example#2: - In the chart given below Mars, which is the lord of 9th house (Trikon) is present in the first house & lord of first house (Kendra as well as Trikon) Sun is present in the 9th house due to which there is a Rajyog due to exchange of zodiacs between Sun & Mars. Similarly there is a Rajyog between Jupiter & Venus. Jupiter is the lord of 5th house (Trikon) & Venus is the lord of 10th house (Kendra) & both planets have exchanged zodiacs.
Example#3: - In the chart given below Mars (lord of 9th house as well as that of 4th house) and Sun (lord of first house) are observing each other due to which there is a Rajyog due to bidirectional vision between them. Similrarly Jupiter & Venus are constituting a Rajyog. Jupiter is the lord of 5th house (Trikon) & Venus is the lord of 10th house (Kendra).