A Guidance Stone is an ancient Sheikah device which interfaces with the Sheikah Slate in Breath of the Wild. Various uses of the Guidance Stones in Hyrule include activating hidden Runes within the Sheikah Slate and adding province maps to the Sheikah Slate.
List of Guidance Stones and functions in Hyrule
Great Plateau Shrines
Each of the first four Shrines Link must complete to advance beyond the Great Plateau contains a Guidance Stone. Each unlocks one of the four fundamental runes of Link's Sheikah Slate - Magnesis, Stasis, Cryonis and Remote Bombs. Each of these Guidance Stones is active as soon as Link encounters it.
Atop each Sheikah Tower a Guidance Stone may be found, which when activated scans the nearby province for terrain data and adds it to Link's map. Each of these Guidance Stones is active as soon as Link encounters it.
Hateno Ancient Tech Lab
Within the Hateno Ancient Tech Lab, found in the far east portion of Hateno Village, a Guidance Stone is found. The stone was originally located within Hyrule Castle before it was moved to its present location. The stone may be used with Purah's aid to unlock and upgrade additional runes of Link's Sheikah Slate. This Guidance Stone's furnace must be activated with a Blue Flame before it can be used.
The removal of this stone from Hyrule Castle is implied to have deprived the castle of its Travel Gate.
Akkala Ancient Tech Lab
A Guidance Stone is found inside the Akkala Ancient Tech Lab in northeast Akkala, however it was converted into a robot by Robbie, whom he calls Cherry. With the proper materials (and a "donation" of funds to the lab), Cherry is able to produce a variety of ancient arms and armors for Link. This Guidance Stone must be activated with a Blue Flame before it can be used.
There are two guidance stones on each Divine Beast. The first stone Link encounters is at the entrance - and registers the Divine Beast as a travel gate. The second provides Link with a map of the Divine Beast.