A Sheikah Tower is a tall structure upon which a Guidance Stone is placed. Their function is to provide Link with map information of the current province, which is done by placing the Sheikah Slate in the Guidance Stone pedestal, whilst also acting as an excellent viewing platform for the surrounding areas. They are grey and gold, with a regal design which bears the Sheikah symbol in fluorescent blue. They extend many meters into the sky, which makes it rather difficult for any monsters to ascend the towers.
Once activated, Link may use them as a fast-travel point any time he is able to Warp.
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 - which had been unearthed in the attack of Calamity Ganon 100 years earlier. 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.
At the beginning of the game, the Sheikah Towers lie dormant underground, until they have been activated by Link. This is done by searching for the activation pedestal in the Great Plateau; it's location being present on Link's Sheikah Slate after speaking with the Old Man. After the Sheikah Slate has been placed in the pedestal, all of the towers can be seen rising from below in a short cut-scene. Link will now be able to climb all of the towers.