What it is
The name suggests thirty parts, but in Parāśara's method each sign has only five parts, and they are unequal. They belong to five planets: Mars, Saturn, Jupiter, Mercury and Venus. The Sun, Moon, Rahu and Ketu get no part in it.
How it is calculated
Rule: in an odd sign, 0–5° Aries (Mars), 5–10° Aquarius (Saturn), 10–18° Sagittarius (Jupiter), 18–25° Gemini (Mercury), 25–30° Libra (Venus). In an even sign the order is reversed: 0–5° Taurus (Venus), 5–12° Virgo (Mercury), 12–20° Pisces (Jupiter), 20–25° Capricorn (Saturn), 25–30° Scorpio (Mars).
Example: a planet is at Aries 17°. Aries is odd and 17° lies between 10° and 18°, so in D30 the planet is in Sagittarius, Jupiter's part. At Taurus 17°, an even sign, the 12–20° part would be Pisces (Jupiter).
What tradition looks at in it
Parāśara links the trimshamsha to the consideration of hardships. ePandit puts it calmly: life tests everyone, and tradition reads this varga for a glimpse of how a person meets those tests, through patience, understanding, courage or conversation. Tradition also reads character and resolve in it. The lord of the 6th house and the D30 ascendant are looked at.
How much the birth time matters
On average the ascendant moves about 1 degree every 4 minutes; some signs rise faster, some slower. One part of D30 is 5° to 8°, so a difference of roughly 20–30 minutes in birth time can change the D30 ascendant. The Moon crosses one part in about 9 to 15 hours. In your chart, ePandit shows next to every varga how many minutes of birth-time change would shift its ascendant. Because the parts are unequal, the sensitivity depends on how close the ascendant is to a boundary; ePandit shows this in minutes.
How to read it calmly
Read D30 as a second look, not a verdict. Tradition first studies the 6th house and its lord in the main chart (D1), then the running dasha. D30 only helps check whether the same theme shows up here as well. A planet's sign in this varga decides nothing on its own. No list of fears comes from this varga. Tests come into every life, and tradition reads it as the way one learns and moves forward through them.