An examlpe of Einstein.
To explain the equivalence of gravity and inertia Einstein considered the example of a lift in free fall (Unlike ordinary lifts, a lift in free fall does not move at constant velocity but accelerates all the time, like an object falling through vacuum) and imagined a tennis ball placed in its centre. To simplify the argument he neglected friction inside the lift by assuming conditions of vaccum.
Because both the lift and the tennis ball are under the influence of the Earth's gravity, both will fall with the same acceleration. Relative to each other they will remain in the same position, and the ball will appear to float in the air.
If the lift is now given an additional acceleration by being pushed downwards, the inertia of the ball (which does not experience the push) will cause it to move towards the ceiling of the lift. It appears to experience a gravity field in which the force of gracity is directed upwards.
Likewise, if the movement of the lift is slowed down by applying the brakes, this is equivalent to an additional acceleration upwards. The inertia of the ball, which does not experience the change in the lift's state of movement, will cause it to move towards the floor. It appears to experience a gravity field in which the force of gravity is directed downwards. (Newtonian mechanics would say that the gravity of the Earth causes it to do that.)
Whether the force that appears to be acting on the ball is called gravity or inertia does not make a difference to the result. In the general relativity theory gravity is only one form of inertia.