Reverse flash was an incel loser who wanted to be a hero but had no powers so he tried to re-create the speed force. He succeeded almost.
He created a "dark speed-force" which instead of being enhanced by hope and compassion was enhanced by anger and hate.
The comics describe dark speed force as the opposite of the Flash's speed force but that is not exactly true.
A more accurate name for him would be the Inverse Flash because the two frequencies of speed force cancel each other out.
When one uses the speed force it weakens the other.
This is why Reverse-Flash can limit Barry's upper speed-threshold by continuing to move fast.
Because the speed force is infinite neither can draw on enough of it to completly de-power the other but they can slow each other down.