Witcher 3 places of power are stones on the map where Geralt gains a ability point for the first time and a temporary stronger sign. With the Blood and Wine expansion there are thirty in total. The regions are White Orchard, Velen, Novigrad, Skellige, and Toussaint.
What places of power do
Hold the button at a stone until an ability point appears. Each stone works only once. You also get a short boost to one sign (Aard, Igni, Yrden, Quen, or Axii). The skill tree fills slowly at the start, so activate places of power as you go and build your signs from that.
The base game has twenty-five stones. Toussaint in Blood and Wine adds five. One place of power sits in Kaer Morhen and one appears during the Through Time and Space quest. Hearts of Stone adds none.
White Orchard
In the starting area White Orchard there are six places of power. They sit close together, so you can grab them early before heading to Velen. A detailed map with yellow markers and where to watch for the griffin nest is in the White Orchard places of power guide.
Velen
In Velen there are six places of power scattered across No Man’s Land. Some only show up during the main story (for example Wandering in the Dark or Bald Mountain). The region is large, so charge stones on the way so you do not have to circle the whole map at the end. A full list and area context is in Velen places of power.
Novigrad
Near Novigrad there are only two places of power. One lies on the northwest edge of the city by the temple, the other on the cliffs north of the Vegelbud estate. Map and access are in Novigrad places of power.
Skellige
On the Skellige islands there are nine places of power, more than any other region. You find them on Ard Skellig, Spikeroog, and Faroe, often on peaks, by the coast, or in caves. Some are guarded by monsters or open only during a quest. An island overview is in Skellige places of power.
Toussaint
In Toussaint from Blood and Wine there are five places of power, one for each sign. They sit by ruins, vineyards, swamps, and a monster den near Beauclair. Some need Aard on a wall or a descent into a cave. The regional map is in Toussaint places of power.
Summary
Charge stones in whatever region you are playing. Start with White Orchard, then Velen and Novigrad, then Skellige, and Toussaint once you reach the expansion. The big picture of the game is in the Witcher 3 review. More guides are on the Witcher 3 hub.