system
Given the following post, respond as if you're commenting on Reddit:
human
[DC comics] What is the speedforce?

Everytime someone asks a a question about the Flash, like how he can see things when running at the speed of light, or how come his hand doesn't brake when he punches things, the answer is always the speedforce. So, to a newcomer to the Comics world, what is the speedforce?
gpt
Quasi-mystical handwave to justify any and all speedster feats that has evolved with about 20 years of stories.  

1. It starts as a power source.
2. Then in becomes a semi-sentient force that grants ESP, additional powers, and which favors a particular line of speedsters.
3. Then it becomes a barrier (like the sound, light, or time barriers) which, once broken through, allows access to speedster afterlife, and where villains can be disposed of without technically killing them.
4. Then it became a mess of incoherent bad techno-babble (that didn't even bother to use fictional scientific terms to obscure the counter-factual nonsense, like "negative speed" fundamentally misunderstanding that speed is an amplitude that's always positive).
5. Then it was mercifully rebooted to excise most of 4 and the afterlife aspect (while still serving as a place to dump villains).

It's practically a character unto itself.  At the end of the day, Flash is a modern Mercury only we're uncomfortable making our heroes magic-based gods, but we can't reconcile those kinds of powers and constraints with science... so the Speed Force is a stop-gap that allows us the fiction we want without having to worry about the scientific explanation or having to cede that it is magic.

Much like The Force lets us have Space Wizards and Knights in Star Wars without having to resort to spells and the occult.