The zodiac is an imaginary "belt" or "roadway" which circles the heavens. The Sun, Moon, and all the planets always stay within this belt. Physically, this is just a consequence of the fact that all the major bodies in the solar system orbit the Sun in more-or-less the same plane. (Imagine the spinning Sun. Put an equatorial line on the Sun just as we do on globes of the Earth. All the planets orbit the Sun directly above this equator, plus or minus a few degrees.) So, as seen from the Earth, it is impossible for any of the heavenly bodies to ever move very far from the zodiac.The twelve constellations which lie across the zodiac are known as the signs of the zodiac...and it is scarcely a coincidence that we also happen to have 12 months in a year. A heavenly body is said to be "in" a certain sign of the zodiac when its position in the sky places it inside the corresponding constellation. In the diagram at right, the Sun is "in" the constellation Leo, because as seen from the Earth, the Sun is between us and the stars that make up Leo. The dates which are traditionally associated with the signs of the zodiac (the ones you look up when you want to know what your sign is, because you were born on that day) are the dates that the Sun is supposed to be in the relevant constellation.
I say "supposed", because alas. The dates astrologers use were accurate maybe 3000 years ago, but since then the precession of the equinoxes has rotated the direction of the Earth's tilt, and now the Sun's progression through the zodiac does not correspond especially well to the dates that are still published every month in every newspaper's astrology column. (They are off by about two weeks.)
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