The fundamental postulate of A. Einstein's general theory of relativity, that posits acceleration is indistinguishable fundamentally from a gravitational field. In other terms, if you are in an elevator that is utterly sealed & protected from the outside, so that you cannot "peek outside," then if you sense a force (weight), fundamentally it is impossible for you to say whether the elevator is exist in a gravitational field, or whether the elevator has rockets association to it and is accelerating "upward."
Even though that in practical situations say, sitting in a closed room it would be probable to find out whether the acceleration felt was because of uniform thrust or gravitation (say, through measuring the gradient of the field; if nonzero, it would mention a gravitational field rather than thrust); though, such differences could be made randomly small. The idea of the equivalence principle is that it behaves around the vicinity of a point, instead of over macroscopic distances. It would be not possible to say whether or not a given (arbitrary) acceleration field was caused through thrust or gravitation by the employ of physics alone.
The equivalence principle predicts interesting general relativistic effects because not only are the two identical to human observers, however also to the Universe as well any effect that occur when an observer is accelerating has to also occur in a gravitational field, & vice versa.