The Speed Force was a mysterious energy field that served as the source of the Flash's superspeed powers.
By approaching near-light speeds, the Flash was able to tap into the Speed Force's energies to defeat the Luthor/Brainiac hybrid, but he was nearly pulled into the Force as a result. Although his teammates were able to free him, the Flash claimed that he could never go as fast as he had again, or risk being permanently drawn into the Speed Force.
In the DC comics continuity, the Speed Force is the energy field which powers all speedsters (individuals who can achieve super-speed). Several speedsters have been absorbed into the Speed Force, however, when they achieved speeds that exceeded their capabilities. Wally West's near absorption into the Speed Force was similar to the death of Barry Allen, Wally's uncle and predecessor as the Flash, when he destroyed antimatter cannon created by the Anti-Monitor during the First Crisis by running around it and causing it to explode.
Examples of speedsters drawn into the Speed Force are Johnny Quick and Max Mercury, both having merged with it.