What is Mangal Dosha?
Mangal Dosha, or Manglik dosha, is said to arise when Mars sits in the 1st, 4th, 7th, 8th or 12th house of the birth chart (some South Indian traditions also count the 2nd). It is checked from the lagna and, by many astrologers, also from the Moon. The belief is that Mars brings extra heat and intensity into partnership, which is why Manglik charts are traditionally matched with other Manglik charts.
Cancellations matter
Tradition lists many exceptions and we apply the commonly accepted ones: Mars in its own sign (Aries, Scorpio) or exalted (Capricorn); Mars in certain house-and-sign combinations (the 1st in Aries, the 4th in Scorpio, the 7th in Capricorn, the 8th in Cancer, the 12th in Sagittarius); and Mars joined or aspected by Jupiter.
Plenty of people have Mangal Dosha, roughly four in ten by the strict rule, so it is far more common than its reputation suggests. It is one factor, never a reason on its own to reject a match. Read it together with the Guna Milan score and the rest of the charts.