Ayanamsa is the number of degrees to subtract to movable longitudes (given by any standard ephemeris) to get the fixed longitudes, that is to say longitudes from a heaven point said to be zero Aries.
A sign is then one of the twelve sectors of 30° from that point (30° calculate onto the ecliptic circle). Ayanamsa or precessional distance exists because the vernal point is not fixed with respect to stars. The vernal point is the zero for astronomical calculations, needed to compute the longitude of planets.
The exact value of the Ayanamsa is not known, because of various motions making difficult to say when the vernal point was the same place as the beginning of the Aries constellation. Hindus think this time was 397 after Christ, but if we take this approach, Spica become set at the beginning of Libra, which is quite strange for the main star of Virgo. Jacques Dorsan says that the year of concordance was 228 after Christ. Opinion of the author of the application is that the beginning of the zodiac is star SAO92659.
So be careful with this value, and be aware that small change to it completely makes Navamsa horoscope a new one. Navamsas are heavily used by Hindus. Lunar periods are also sensitive to the Ayanamsa value.
See also: