Two related concepts run across Polynesia, and both have been badly served by their English borrowings.
Mana is efficacy: power that shows itself in results. It is held by gods, by ancestors, by chiefs, by objects, and by people who are good at things, and it is demonstrated rather than asserted, a successful voyage, a productive garden, a won battle, a fishhook that catches. Because it is inherited through senior descent lines and also earned, it explains both why chiefly rank matters and why rank can be lost. The New Age use of the word to mean a generic energy has little to do with this.
Tapu is the restriction that protects and results from mana. A person, place, object or activity that is tapu is set apart and may not be approached or handled without the right standing or the right rite; its opposite, noa, is the ordinary or freed state, and rituals exist to move things between them. The English word taboo derives from tapu, encountered by Cook's expedition in Tonga in 1777, and it has since drifted a long way toward simply meaning forbidden, which loses the sacredness that is half the meaning.
The two concepts did practical work. Tapu on a fishing ground or a stand of trees closed it for a season and functioned as conservation, whatever else it was. Tapu regulated contact between ranks, between men and women, and around birth, death, warfare and food. Breaking it was expected to produce harm without anyone needing to enforce it, though enforcement also existed.
Both survive in current use. Mana appears in ordinary New Zealand English and in political speech about standing and authority; tapu still governs conduct around death, burial grounds and food in many communities, including Christian ones, and is written into some public protocol.