In physics, if there is no way to distinguish two things, they are the same.
If you are in a box that is accelerating, what you experience is being pushed to one side of the box.
You would intuitively assign that side the special label "floor", and how hard you press into it would be your "weight".
This is also what you experience standing in a box on the Earth, but not when you are in a box in freefall... it follows that standing on the Earth is not distinguishable from a situation where you are accelerating while freefall is not distinguishable from the situation where you are not accelerating.
That's not all there is to it - but it's a starting point.