[twentyfiveoh1]

I am not an expert at all but I believe the rough idea is that in General Relativity, everything follows a geodesic. The straightest possible path through curved spacetime.

They use that to explain why you don't feel acceleration when you jump off a building or out of a plane.

They say you DO feel it when you are standing, because the earth is impeding you and pushing you away from the geodesic you naturally want to follow.

So it is counterintuitive. the standing still person is being accelerated/pushed. The freefall person is free.

John wheeler said "Spacetime tells matter how to move; matter tells spacetime how to curve". I think it explains the circular nature.

Mass curves spacetime, that changes how mass moves , mass moving to a new position changes the curvature, that changes how the mass will move next. The dimple in the mattress staying with the mass.

[hollerith]

You've got it right. Our normal environment is an accelerating reference frame. The resting frame (i.e., free fall) is easier to analyze using physics, but is unfamiliar (indeed tend to feel alarming) to us.