Random State Scramble
Rather than generate a sequence of random moves (the traditional way to scramble), a random-state scramble simply directly assembles the cube in a scrambled state. Typically a solver is used to convert the scramble into a sequence of moves, to allow a solver to easily apply it to their cube. Random-state scrambles guarantee that the cube will be in a completely random state, with no statistical bias towards easy solutions.
Human Scrambling Method
Physically attempting to random-state scramble a cube would involve taking it to pieces and re-assembling it in random order. Because of the Laws of the cube, there is only a 1 in 12 chance that a cube assembled in this way will be solvable. In order to do this successfully the laws of the cube need to be understood, and final cubies placed in such a way as to to comply with them.