If I had to explain the Speed Force, I'd say it's.....complicated. If you asked me to elaborate, I'd say it's
very complicated. But if you twisted my arm and made me give you my notes, I'd say it's some facet of extradimensional reality that's both intrinsic to time-space as we know it while also being separate from it. Dr. Wells has all these theories about Hyperspace, Hypertime, the so-called 'Dark Energy' that's causing the expansion of all matter in the universe, and the Speed Force seems to be tied into all of them. For practical purposes, I can convert my body and the world around me into a sort of 'hyper-matter,' allowing me to move at relativistic speeds without all the messy business of relativity-- not to mention the physical limitations of air friction, escape velocity, and getting bugs in my mouth. The deeper I delve into it, the more of myself I convert, the faster I go, and the crazier stuff I can do.
It's useful, sure, but tapping into the Speed Force is also dangerous. Usually I sort of dip my toes into it, just enough to go about Mach 10 or so without accidentally peeling my skin off and launching my skeleton into orbit. Occasionally, I'll have to dig deeper, going larger and larger fractions of the speed of light, and when I do that, things get....weird. Time distorts, causality starts to break down, not to mention it leaves me feeling like hell in the aftermath. The deeper I go, the harder it is to come back to the real world. If I go all the way with it, completely convert over to the Speed Force, I could theoretically be everywhere all at once, but I don't think I'd ever come back.