The lagna is the degree of the zodiac rising on the eastern horizon at the moment of birth. It is the reference point from which the twelve bhavas are counted, and in most Indian practice its sign lord — the lagna lord — is read as the single most important graha in the chart.
It is also the reason birth time matters. The whole zodiac rises in twenty-four hours, so a new sign comes up roughly every two hours and a new degree every four minutes. Two children born ninety minutes apart in the same hospital may share every planetary position to within a fraction of a degree and still have different lagnas, which reassigns all twelve houses and produces two quite different readings. This is the most common practical failure in Indian astrology: birth times are frequently recorded to the nearest half hour, remembered rather than documented, or noted in local time with no record of what that meant that year.
Compare the Moon, which moves about half a degree an hour, and the Sun, which takes a month per sign. A birth date alone fixes the Sun's sign and roughly fixes the Moon's. It fixes nothing about the lagna.
Anyone whose birth time is uncertain should treat house-based statements with corresponding caution, and treat any reading that does not ask about birth-time accuracy as incomplete.