In Vedic astrology, Manglik Dosha (also known as Kuja Dosha) occurs when the planet Mars (Mangal) is in the 1st, 4th, 7th, 8th, or 12th house of an individual's birth chart. This placement is believed to bestow fiery, assertive energy, which can manifest as challenges in marriage and relationships if not balanced. 
Here's a more detailed explanation:
What is it?
Manglik Dosha is an astrological combination where Mars (Mangal) is positioned in specific houses of a birth chart. 
Which houses?
The 1st, 4th, 7th, 8th, and 12th houses are considered problematic for Manglik Dosha. 
Why is it significant?
Mars, known as the God of war, is a fiery and aggressive planet. Its placement in these houses can lead to conflicts, misunderstandings, and challenges in marriage and relationships. 
Impact on Marriage:
Individuals with Manglik Dosha are believed to face difficulties or instabilities in their married lives. This can manifest as delays in marriage, discord, health issues, or financial setbacks. 
Remedies:
Astrologers suggest various remedies to mitigate Manglik Dosha, including marriage to a Manglik person, performing specific pujas and rituals, or taking proactive measures to channel the fiery energy constructively.