Is the Universe curved, or flat?

I don't know what they mean about curved universe? Or how they tell it's flat? The universe is quantized like all of the marbles in a jar. How do you tell if the marbles in a jar make up a curve? If the jar is spherical do the marbles look different to if the jar is a cube?

Surely the way the marbles stack looks the same from inside the centre of the jar?

This confuses me.

Pincho Paxton