A zodiac sign is a slice of the year, and Capricorn is the tenth of the twelve, sitting between Sagittarius and Aquarius. Its picture is a goat, and in astrology it is said to be ruled by the planet Saturn. In this meaning the word is uncountable and usually has no the or a in front of it: her sign is Capricorn. Note that the dates depend on the system: western horoscopes use about December 22 to January 20, while sidereal astrology places it later, around January 15 to February 14. Whether any of it works is not the dictionary's claim — the word is simply what English calls that slice.

Here the name of the sign is used for the people under it. This one is countable, so it takes a or the and has a plural: she's a Capricorn, three Capricorns in one office. That little a is the whole difference between the two meanings, which is why it is worth watching. In casual talk people attach characters to it — Capricorns are said to be hard-working and serious — and the clipped form Cap turns up in chat and in profiles.