As inhabitants of the Earth, we are part of the solar system in which eight other Planets (Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto) orbit around our Sun.
In astrology, we look at this solar system and imagine that the Earth is the center of this system.
This so-called geocentric world view was long ago considered a reality until it was slowly replaced by the heliocentric (with the Sun as the center) in the 16th century.
In astrology we work with the planets of our solar system as well as with the Sun, Moon, and sometimes also with planetoids (f.e. Chiron) and symbolic points which are determined purely by calculation (such as lunar nodes or Lilith).
For the sake of simplicity, we usually speak of planets and also mean the Sun, Moon, and, for example, Chiron and Lilith.
Symbolically, the planets in the horoscope stand for our various basic needs, energies, or parts of our personality that we want to express.