The Speed Force is an extra-dimensional source of energy where Speedsters draw their power to run extraordinarily fast and to utilize a number of abilities including time travel and travelling between dimensions. The Speed Force also acts as a sort of purgatory-like realm for speedsters who when killed appear inside the Speed Force and are offered the chance to leave but without their speed, but will restore their power if they prove themselves worthy enough to wield it again. The Speed Force also has enforcers called Time Wraiths to pursue those who use time travel for selfish or foolish reasons, either killing them to prevent further disruption of the timeline, or to imprison them within the Speed Force itself as punishment. The Speed Force is also somewhat sentient, manifesting itself in the forms of those closest to speedsters within the Speed Force to communicate with them. The Speed Force itself claims to have been around since the Big Bang itself, and will exist until the end of time itself.
- The information here was copied from the Arrow Wikia, all credit goes there.