A Sheikah Tower is a tall structure upon which a Guidance Stone is placed.
The Sheikah Towers were constructed by the ancient Sheikah of Hyrule over 10,000 years ago. Buried beneath the earth, they had long since been forgotten by the people of Hyrule until Link discovered the top of the tower in the Great Plateau. After having activated the tower using his Sheikah Slate, the buried tower upon which he stood, along with the other towers across Hyrule, suddenly rose from the earth.
Holding the Sheikah Slate to the Guidance Stones at the top of each tower causes the stone to then scan the landscape of the region of Hyrule in which it is located. This data is then condensed and transferred to the Sheikah Slate, allowing Link to view a geographical map of the region at will.