The Speed Force is the extradimensional power from which all speedsters draw their ability from. Motion in the universe is a form of computation. In other words, when you pour coffee into your cup in the morning, that action is performing a material calculation, an algorithm, based on the operating system of the universe, the laws of nature (gravity and the other forces). Now, the fastest "processing" speed is the speed of light. As relative motion approaches the speed of light, then reality comes close to breaking down. Space and time warp and matter comes very close to breaking down. The same thing happens when matter approaches the temperature of absolute zero which denotes no molecular motion at all. Matter at that point condenses into "superatoms" which are basically undifferentiated information, potential but unused data in the processor of the universe.
The Speed Force functions as is a space-time "processor" designed to "upgrade" the local environment of a speedster. Basically, it increases the processing speed around the speedster so that he or she can approach the speed of light without exhibiting relativistic effects. This also accounts for the protective friction "aura" that prevents the speedster from bursting into flames. The Speed Force's computation creates a tight local field around the speedster's body that makes all molecular contact appear to be no faster than if the speedster was running at normal speed. This also creates an extremely unique perceptual effect in the speedster's mind. While running, he can both experience the relative speed at which he appears to be moving (he can accelerate to near the speed of light) while at the same time he can perceive local events as if they are standing still. Anything within say a six foot radius would appear to slow down the faster he or she ran while the surrounding environment outside would appear to be rushing by. The speedsters are chosen by their inborn ability to handle this perceptual dissonance.