There are various places of power that grant a bonus to Geralt distributed around the map. The vicinity of a place of power is indicated by a characteristic sound effect, if you activate your medaillon you can see them as a red mist. If you step into this red mist while the medaillon is active, you are granted one of the possible bonuses.
I didn't find any place where the exact nature of each bonus was noted. How many different places of power are there and what are the bonuses they grant you?