Manglik dosha is a belief in Hindu astrology that a person with this dosha is inauspicious for their wedding and will cause their spouse to die early. There are many reasons given for why this is so, but the most common one is that the planet Mars is considered to be a planet of war and destruction, and so when it is in close alignment with a person's natal chart, it is thought to cause problems in the marriage.
Manglik dosha is said to be present in around 32% of the population, and so it is considered to be quite a common thing. It is not always easy to tell if someone has this dosha, as it is often only revealed through a detailed analysis of their natal chart. However, there are some general indications that someone may be Manglik, such as being born under the sign of Aries, having Mars in their 7th house, or having a strong affiliation with the planet Mars.