The Speed Force

The Speed Force is a physical entity that provides Barry with his super speed abilities. While not taking physical form, it still communicates via speech.

History
Man in the Mirror

After Barry defeats Mirror Master, he enters a random portal only to find himself in the Speed Force. The Speed Force tells him that it provides Barry with his speed. Barry doesn't care, wanting to go home. While The Speed Force releases Barry back into the physical realm, he warns him that new foes are coming, and that if The Flash dies, it dies.

Behind the scenes
If there's anything you'd like to say that's not derived from episodes, but maybe comes from interviews that actors or crew gave, use this section.