After you leave the tutorial in The Elder Scrolls Online, you find yourself in the world of Tamriel. Wherever you end up, upon exploring, you will sooner or later notice a glowy ray emitted from what looks like a crystal. These are called Skyshards.
Unless you happen to travel to the mainland first and do the Soulless story quest first (or the main story quest), you won't get an explanation on what a Skyshard is. The Soulless quest used to be the tutorial quest long time ago.
Why are Skyshards useful?
Skyshards are meant to provide you with a skill point after you have collected three of them. In addition, upon finding all in an area, such as a region, you will be granted an Achievement. If you have collected all them, you will receive a new dye with which you can colour your costumes or clothes, called Aetherial Blue.
Since you also have to go into solo- and groupdelves in order to find some Skyshards, you will also get some other achievements, unlocking more colours and titles!
In the total 32 regions you can find 452 Skyshards. If we include the one from the Soulless quest this makes a total of 453 Skyshards you can find, which accumulates to 151 Skill points!