Star charts were astronomical maps of constellations.
The First Doctor possessed an astral map (TV: The Web Planet) and later had paper charts. The Fifth Doctor tried to teach Nyssa and Tegan to read them just before their second encounter with the Mara. (TV: Dragonfire, Snakedance)
Voyager owned star charts that showed how to access "the dimension of death". Astrolabus had them tattooed onto his body and then ran away. Thinking them stolen, Voyager instructed the Sixth Doctor to reclaim them. He eventually caught up with Astrolabus and Voyager killed him. (COMIC: Voyager, Once Upon a Time Lord)