Lifestream
The Lifestream is a current of Aether that flows through all things, including the planet and all living beings. When something is born, it is given aether from the Lifestream, and the current will flow through them. This current is what gives living beings the ability to use Magic. When a living being dies, their aether returns to the Lifestream, where their memories and personality eventually fade. This cycle continues in perpetuity, in an endless cycle of life and death. The Mothercrystal, deep beneath Hydaelyn's surface, is said to the the center of the Lifestream.
Although this flow of aether is the source of all life, it has dire consequences for anyone who enters into it in their corporeal form. The abundance of aether can cause massive instabilities within the body's flow, and can lead to devastating effects, and eventually death, with the being slowly breaking down and their consciousness drifting through the Lifestream, where it will eventually fade. The variety of effects floating in the Lifestream can have are wide and varied, from an inability to use magic, as observed in Thancred, or the ability to see the flow of Aether, as with Y'shtola (this did seem to come at the cost of her losing her ability to see normally, however, it is unclear if the two are connected). While there is a chance that the effects can be beneficial, it is far more likely that the effects will be incredibly life-threatening.
While entering into the Lifestream can be dangerous, the people of Eorzea have mastered magic that allows them to reduce themselves down to their aether and maintain their consciousness, while using the Lifestream as a method of travel, using Aetherytes as waypoints. This method of travel allows near instantaneous travel from anywhere close by like a town, to as far away as another continent, or even the Moon.