The flow state is a mental state. It is achieved when one is in deep concentration, literally a state of hyper-productivity. It’s not very hard to enter, but once you do, you can do wonders. World’s biggest scientific discoveries are done when the scientists were in the flow state. Inventions were invented in the flow state.
When you are in the Flow state, time seems to go quicker and productivity increases by tons. You can get more done and can feel like your capabilities have increased exponentially, that’s what happens when you set foot in the grounds of the flow state.
It is not necessarily hard to enter. You can enter the flow state by first of all, completely eliminating distractions. Then, start the activity and don’t stop. When you get so immersed, you’ll feel like you are in another world. Then you won’t stop unless you’ve felt like you’ve achieved what you wanted, or you get distracted, say, by looking at your phone, essentially breaking from the flow state.
To explain the flow state in simple words, it’s that feeling when you’re “in the zone” that you feel like you’re unstoppable and can do something in a much better way.
You might have gone into the flow state without realizing it, we all have, most probably, and that’s when we have done what we thought weren’t possible by us. One example I can think of is while writing an exam. That’s a good example of when you can enter into the flow state, maybe not completely, but partially, since you’ll still be sort of aware of your surroundings.