In reference to The Flash TV Series Season 2 Episode 20:
In the end, we see that Barry Allen doesn't exist in physical World anymore. But his soul is trapped in Speed Force (Got this from YouTube channel : pagey) .
What does 'trapped in speed force' actually means?
PS: I am not a comic guy...
The Speed Force is a energy field which permeates all of space-time in the DC Universe and allows many of DC's most well known speedsters to defy the laws of physics.
Depending on the quality of a speedster's connection, each will be able to manifest a variety of superhuman abilities related to hyper-accelerated activity.
Almost all speedsters who can connect to the Speed Force can move at what is commonly called super-speed which includes, faster ground movement, heightened reflexes, heightened temporal perception and increased reaction time.
They also have increased prioperception - an awareness of their body's physical position, movement and a perfect awareness of what is around them. This is necessary for speedsters to prevent accidents and bumping into things at high speeds.
The source of all human speedsters superhuman abilities is their connection to the Speed Force. Believed to be a subset of the Source, a cosmic repository of energies which power all superhuman ability, this power is uniquely reserved for and accessibly by Human speedsters and anyone utilizing technology which can also access this energy field.
When the Flash is running, the Speed Force surrounds and envelops him in a protective aura. He can extend that aura to people he is carrying, so they are also safe from flying debris, wind burn, friction and other associated speed related issues.
All of these energies are bled into the Speed Force and have no effect on the Flash or his surroundings, unless he wants them to. He can withdraw speed from other things or people and add that speed to his own.
Most importantly, the Speed Force is augmenting the Flash's brain and his perception, speeding up his thinking process, helping him to organize and coordinate his movements, extending his awareness of his body so he doesn't touch anyone at the speeds he is moving. It would be disastrous.
Once the Flash starts moving at speeds faster than sound, the Speed Force goes into overdrive. Once the Flash is opening up his speed, he is faster than a bullet. To outrun a bullet he needs to be able to reach speeds of up to 1,700 miles per hour!
His reflex time is so good and he is so fast, he can, using the Speed Force, remove the momentum and energy from a bullet and pluck it out of the air, like a raindrop. Only the Speed Force would allow him to steal the momentum and energy and add it to his own.
When speedsters in the DC Universe travel as fast as they are capable of, it is possible for them to brush up against the Speed Barrier, a protective field surrounding the Speed Force. Hit hard enough, a speedster can skip forward through time in an uncontrolled fashion. See: Max Mercury.
If a speedster is fast enough and in sync with his speed powers, he can be drawn into the Speed Force itself as a kind of Heaven or Nirvana for Speedsters. They become one with the speed energy and are reluctant to return to the land of the living.
The Speed Force has been used as a prison to hold renegade speedsters such as Savitar and the incredibly dangerous Superboy Prime.
There has been a precedent set for speedsters escaping the Speed Force if they have a connection to a loved one or a singular purpose powerful enough to force them to return to Earth.
Wally West (in the comics) was the first speedster who was able to enter the Speed Force dimension and willingly return due to his intense feelings for Linda Park, his fiancée at the time. Wally also used the Speed Force as a kind of prison pushing his enemy into it.
Later, Barry Allen has managed to return and the villainous Savitar was able to escape as well.
