Whenever Mars is situated in 1,2,4,7,8,12th house of a Rashi or lunar chart (Moon chart) it is called as Manglik.
There is nothing wrong in being a Manglik. Mangliks are normal people. Most importantly, Mangliks should realize that being a Manglik is not a matter to be afraid of; it is not the end of the world for them. Matters pertaining to the intensity of the Manglik Dosha and its cancellation play major roles and these are to be studied deeply

Source: google

Mangal Dosha" is an astrological condition that occurs if Mars is in the 1st, 4th, 7th, 8th, or 12th house of the Vedic astrology lunar chart. A person born in the presence of this condition is termed a manglik.
Mars holds a very important planet in our horoscopes. It is the planet of action and our capacity to project emotion - its nature is that of a brave warrior, one who is also ruthless. A correct balance of Martian energy is the most desirable in all lives - but unfortunately, only a lucky few possess this dynamic balance. Mars is the planet of emotional excitability, which can become violence. Generally speaking, Venus is the planet of emotion sensitivity, which can become refinement while Mars represents the malefic side of our emotions.
On the positive side, a strong Mars is necessary to give us the energy, independence, will and self-confidence to carry out our endeavors. Without this we would have no real interest, passion or motivation to carry out anything to an end and ...more

If Mars is in the 2nd, 4th, 7th, 8th, or 12th house of the Vedic astrology Ascendant chart (In north India, 1st house is also considered in effect of this Dosha). A person born in the presence of this condition is termed a manglik.

Jo mangalwar ko paida hota hai use mangalik kahate hai