CM
Put simply.
In VFR flight you look out of the cockpit window and see the horizon and the ground features and use your visual senses to tell you what is what.
In IMC you can't see the ground and have to rely on the instruments to keep you straight and level, on course and at the correct altitude.
This requires learning and knowing to trust your instruments rather than your senses.
As I said, this is simplistic - in VFR flight you still use your instruments to check speed heading and altitude, but you are not reliant on the instruments, whereas in IMC you are.
Flying VFR into IMC is therefore the transition from one to the other, and can result in accidents, especially among those not fully trained/conversant with relying solely on their instruments.