The tropical year
The tropical year is the time from one spring equinox to the next, about 365.2422 days. It determines the seasons and the calendar year.
What it is
Because the tropical year is shorter than the sidereal year, the seasons move slowly against the stars over centuries. The Gregorian calendar is designed to keep the civil year close to the tropical year.
How it is worked out or read
The site's calculations use the sidereal Sun, so the tropical year is used only for the reference to seasons and in comparing with the civil calendar.
Where it appears on this site
See sidereal year and signs and seasons.
What the tropical year measures
The tropical year is the time from one March equinox to the next, about 365.24219 days. It is the year of the seasons, and it is the year that the Gregorian calendar is designed to follow. The Gregorian rule of leap years, which drops three leap days every four centuries, keeps the civil calendar within about a day of the tropical year over thousands of years.
Because the tropical year is tied to the equinox, and the equinox drifts against the stars, the tropical year is about 20 minutes shorter than the sidereal year. Western astrology uses a zodiac tied to the equinox and so follows the tropical year; Jyotish uses a zodiac tied to the stars and so follows the sidereal year.
Where the tropical year still appears in Jyotish
Even a sidereal system cannot ignore the tropical year, because the seasons, the solstices and the length of daylight all follow it. The ayana, the Sun's northward and southward course, is strictly a tropical idea, since it turns at the solstices. Sunrise and sunset times, which drive the panchang's day, also depend on the Sun's tropical position, because they depend on its declination. So a sidereal panchang still computes tropical quantities in the background.
This is why the site first computes tropical positions and only then subtracts the ayanamsa. The tropical position is what the astronomy gives directly; the sidereal position is a defined offset from it.
The calendar link
Ritu, the six traditional seasons, are sometimes counted from sidereal sankrantis and sometimes from the solstices. Over centuries the two counts drift apart, for the same reason Makar Sankranti has drifted away from the winter solstice. A reader who meets two different dates for the start of a ritu in two almanacs should check which year each almanac uses as its basis. Neither is wrong; they are answering slightly different questions.
A note on precision
The tropical year itself changes very slowly, by a fraction of a second per century, and its length depends a little on which equinox or solstice is used as the reference point. For calendar and chart work these refinements are far below any practical concern. What matters for the reader is the roughly 20-minute difference from the sidereal year and the way it accumulates, about one degree in 72 years, into the ayanamsa.