The Navamsa is the ninth divisional chart: each sign of 30 degrees is cut into nine parts of 3 degrees 20 minutes, and every planet is re-plotted into the sign its part falls in. In the sixteen vargas of Brihat Parashara Hora Shastra ch. 6 it is the chart of marriage and dharma, and it is the one varga examined on almost every reading alongside the birth chart itself.
Divide the 30 degrees of a sign into nine equal parts of 3 degrees 20 minutes each. Which sign the first part maps to depends on the modality of the sign the planet is in: a movable sign starts from itself, a fixed sign from its 9th, and a dual sign from its 5th. From that starting sign the nine parts run forward through the zodiac in order.
One consequence is worth noticing. Nine parts of 3 degrees 20 minutes is exactly one nakshatra pada, so a planet's navamsa and its nakshatra quarter carry the same information seen two ways. That is why the pada shown on a Moon-sign or nakshatra reading and the D9 position of the Moon always agree.
A planet that occupies the same sign in the birth chart and in the navamsa is called vargottama. AstroZivi computes that flag for every graha in the full kundli, because it is the single most-cited piece of information the D9 supplies: the same sign twice is read as a placement that holds its ground, and a chart's vargottama planets are usually the first thing a practitioner notes after the lagna.
The other thing the division does is act as a resolution test. Because a navamsa is 3 degrees 20 minutes wide, a planet near the end of a sign in the D1 can land nine signs away in the D9, and two people born twenty minutes apart can have different navamsa lagnas. The chart therefore separates births that look identical at the level of signs alone.
The tool re-plots all nine grahas plus the ascendant into their navamsa signs and draws the result in the same house format as the birth chart, with the navamsa lagna as the first house. It shows each planet's sign, its house from that lagna, and its dignity in the D9, so exaltation or debilitation that only appears in the division is visible.
What the page does not do is convert a placement into a prediction. BPHS ch. 6 gives the D9 its subject, marriage and dharma, and this page names that subject and stops there. Where AstroZivi does carry a reading for a placement, it ships with a text and a verse behind it; here the honest output is an accurate chart with the sign, house, degree and dignity of every graha.
For educational purposes. Astrological interpretations are traditional and not a substitute for professional advice.