The chakra system reads the body as carrying seven energy centers running from the base of the spine to the crown of the head, each paired with an element, a color, a musical note, a seed sound and an endocrine gland. A reading maps a chart's planetary placements onto those seven centers and describes where the tradition would read energy as gathered, blocked or overflowing.
History
Textual references to centers of subtle energy in the body appear in the Upanishads and are developed at much greater length in later Hindu Tantric literature. The specific seven chakra picture familiar in the West today, arranged from the base of the spine to the crown of the head, owes its now standard form substantially to one text and one translator: the Sat-Chakra-Nirupana, a sixteenth century Sanskrit Tantric work, reached English speaking readers through the British Orientalist Sir John Woodroffe, writing under the pen name Arthur Avalon, whose 1919 translation and study, published as "The Serpent Power", is the direct source most twentieth century and later Western chakra writing draws on, directly or at several removes.
It is worth naming plainly that the wider Tantric textual tradition did not hold to one fixed number of centers before that translation fixed the picture for Western readers: various texts describe five, six, nine or other counts and arrangements rather than uniformly agreeing on seven, so the standard seven chakra scheme is a real but historically particular selection rather than the whole tradition's single settled scheme. Several further layers commonly attached to the system today, correspondences to colors, musical notes and specific endocrine glands, are clearly modern additions, popularized especially from the 1970s New Age movement onward, without deep textual precedent in the Sanskrit sources themselves.