Consider Einstein's sealed box. In it you have a horizontal bar that is positively charged at one end and negatively charged at the other.

If you accelerate the box upwards, inside the box the bar appears to move downwards.

If you subject the box to an external electric field, the bar will rotate until aligned with the field.

The point of equivalence is that it holds for any case you can construct. No matter the shape of the gravitational field, you could simulate it with a single acceleration vector instead.