Jupiter
Jupiter governs expansion, luck and meaning: what a person believes in and where they "get lucky."
Governs
Jupiter describes a person's worldview, their attitude to knowledge and faith, and their inclination toward risk and generosity. In the tradition it is the "greater benefic," a planet whose position is considered favorable almost always.
Jupiter's cycle is about twelve years, roughly a year in each sign. This cycle is the origin of the twelve-year count of years in Eastern astrology: Chinese astronomers counted years by Jupiter's orbit long before the animal zodiac appeared.
In the signs and houses
By sign, Jupiter shows what a person believes in and how they expand: in Sagittarius, through travel and ideas; in Virgo, through skill and order.
By house, it shows where growth comes: in the second, money; in the ninth, education and distant lands; in the eleventh, through community.
Strong and weak placements
Domicile: Sagittarius and Pisces; exaltation: Cancer. Detriment: Gemini and Virgo; fall: Capricorn.
Jupiter returns to its natal position at roughly ages 12, 24, 36 and 48, and astrology considers these years points of expansion. The match with real life stages has a simpler explanation: most people go through transitions at these ages anyway.