Sidereal zodiac
In plain words
The zodiac measured from the stars themselves rather than from the seasons. Choosing it usually moves your Sun back into the sign before the one you grew up calling yours.
In the tradition's terms
Ecliptic longitude measured from a star-fixed origin — the zodiac Indian astrology has always used.
In depth
The origin is fixed by an ayanamsa. Aspects and house placements are unchanged, because both are differences and a difference survives a common shift; only the sign changes, and the sign is what a reader notices.