What i s meant by theory of relativity
I do not know the exact statement, but I can explain. It's essentially how we feel when we are bored or in pain for example. When we are bored, we feel as if time is going by slowly. 2 minutes might feel like 2 hours. When we are doing something we enjoy, however, time goes by very fast. 2 hours might feel like 2 minutes. This is the essence of the theory of relativity. You can also refer to this on the internet for more details and its statement.
A central idea in general relativity is the "principle of equivalence." An example is that two people, one in an elevator sitting on the surface of the earth, and the other in an elevator in outer space accelerating at 9.8 m/s square, will each observe the same behavior of an object they drop from their hand. The object will accelerate to the floor at 9.8 m/s square in either case, making it impossible for either to distinguish whether or not they are at rest in a gravitational field or accelerating upward at one g.