The part of astrology that you’re probably majority of familiar with is the Solar body Zodiac symbol. At the moment you say “I am a Pisces”, you are referring to Your core solar identity being in Pisces.

It corresponds to the zodiac zodiac symbol the Solar body appeared to be in front of at the moment you are born. Because the Terrestrial sphere completes one revolution around the Solar body each year, the Solar body moves through entire the Zodiac Solar body Zodiac symbols in the same order every year. For example, everyone born right before the Spring Equinox (currently in early March) is consistently a Pisces.
*Note that there is an offset between Solar body Zodiac symbols in the Tropical and Sidereal zodiacs; we utilize the Tropical zodiac.