Aragala is a very important concept in vedic astrology. Argala means bolt . Argala of a planet on a house shows the influences that intervene in the affairs of the house. Planets having Rasi Dristi have a small influence and planets having graha dristi  have a more influence of the house when they have argala on the house in question.

A Planet or a house in the 2,4,11th house from a planet or a house causes primary argala on the later.  Benefic planets has benefic argala and malefic planets have malefic argala. Benefic intervention is called as Subha Argala and Malefic intervention is called as Papa Argala.

Virodha Argala shows the obstruction of the primary argala...Planets or houses placed in 12,10,3 has Virodha argala on the house or planets in 2,4,11.

If there are malefic planets in these houses and they causing a argala on the other houses as mentioned above they are considered as Malefic argala.

In the same way planets in the 9th house house has argala on the 5th house and Vice Versa...But if there are more planet in the 3rd house specially malefics placed there it has to be considered as argala instead of Virodha argala.