The fixed stars are the individual named stars used in astrology (Aldebaran, Regulus, Antares, Sirius, Algol and the rest), distinguished from the planets by their apparently unchanging positions relative to one another. In a chart a planet or angle conjunct a notable fixed star is read as taking on that star's character.
Ptolemy's Tetrabiblos assigns each prominent star a planetary nature, Aldebaran is of the nature of Mars, Regulus of Mars and Jupiter, and the medieval Arabic and Latin traditions added magical images and used the brightest stars, the "behenian" stars, in talismanic magic.