the equivalence principle
Einstein called it his happiest thought: a person who falls freely feels no gravity at all. Ride a lift whose cable has snapped and you float; drop a ball and it hangs beside you. Now seal that same lift in deep space and accelerate it upward at 9.8 m/s^2, and everything inside behaves exactly as if it sat on the ground. From inside a small box you cannot tell gravity from acceleration. That single observation is the seed from which all of general relativity grows, and it answers a question Newton left open: why does gravity accelerate every object equally, feather and cannonball alike?
The principle comes in layers. The weak equivalence principle says the inertial mass that resists acceleration in F = m a equals the gravitational mass that feels gravity in F = G M m / r^2, so the mass cancels and all bodies fall with the same acceleration regardless of their composition. This has been tested to about one part in 10^15. The Einstein equivalence principle goes further: in a small enough freely-falling frame, all the laws of physics take their special-relativistic form, and gravity is locally indistinguishable from acceleration. The word 'locally' is doing real work, meaning within a region small enough that gravity does not visibly vary across it.
This is why gravity in general relativity is not a force at all but the geometry of spacetime: because you can always transform it away locally by jumping into free fall, it cannot be a true force field like electromagnetism. A common misconception is that the equivalence principle abolishes gravity entirely. It does not. Over an extended region the field varies, and that variation, the tidal effect, is genuine curvature and cannot be transformed away by any choice of frame. Only the uniform part is fictitious.
Astronauts on the space station float not because gravity is absent there (it is nearly as strong as on the ground) but because the whole station is in continuous free fall around the Earth, an enormous Einstein lift.
Free fall, not the absence of gravity, is what produces weightlessness.
Tidal forces are the escape clause: two nearby freely-falling particles slowly drift together as they fall toward a planet's center, and no choice of frame removes that relative acceleration. It is the true signature of gravity, and it is exactly what curvature measures.