Manglik dosha, also called Mangal dosha, refers to the planet Mars occupying particular houses in a birth chart. It is most often discussed in the context of marriage compatibility.
Which houses count
Traditionally Mars in the 1st, 2nd, 4th, 7th, 8th or 12th house from the ascendant is considered Manglik, though some schools count from the Moon or Venus as well. The strength and sign of Mars change how strongly it is felt.
Many charts carry some form of this placement, and cancellations are common. A careful astrologer weighs the whole chart before drawing any conclusion, rather than reading the dosha in isolation.