Short answer
The Ascendant, or Lagna, is the zodiac sign rising on the eastern horizon at the moment and place of your birth. It becomes your 1st house and fixes the order of all twelve houses. Because a new sign rises roughly every two hours on average, a birth time out by even 15–30 minutes can change the Lagna if you were born near a boundary.
How ChiChakra reads this
The Lagna decides which life area each planet falls into, so it shapes the whole chart. ChiChakra's current forecasts use your Moon sign instead, which doesn't need an exact Lagna, and we never guess a missing birth time.
Getting the time right
Use the birth certificate or hospital record, and check the timezone and any daylight-saving rule in force that year.
If you only know 'early morning' or 'around noon', treat the Lagna as uncertain and read the chart from the Moon sign (Chandra Lagna) instead.
Some astrologers offer birth-time rectification, matching the chart to known life events. Treat the result as an estimate, not a fact.
Good to know
Personal birth-chart readings, including Lagna-based ones, are coming. The waitlist is the place to hear when.