What it describes
A coven raises energy together, usually by chanting or dancing in a circle, and the accumulated effort is pictured as a cone rising from the circle to a point above it, then released towards a single agreed purpose.
Where the idea comes from
The Cone of Power is strongly associated with Gardnerian and Gardnerian-derived Wicca from the mid-twentieth century onwards. Gerald Gardner popularised a claim that a New Forest coven raised a cone against invasion in 1940, sometimes called Operation Cone of Power. Historians treat that account as unverified: it rests on Gardner's own later telling.
Not ancient, and not Celtic
You will see the cone presented as an old Celtic or ancient pagan technique. There is no evidence for that. It belongs to twentieth-century witchcraft, which is quite interesting enough.
Solitary adaptation
The specific cone imagery assumes a group. A solitary practitioner can use the broader idea instead: build focus by repetition, hold it, then release it at a chosen point. Some solitaries keep the cone image; others find it faintly silly on their own, and use rising heat, a held breath, or a single word instead.