The ball in a sheet cloth is a nice two dimensional model.

But I like this example as a three dimensional model.

Imagine a 3 dimensional grid of bungie cords so you have bungie cords going up and down, left and right, towards you and away, such that the bungie cords are connected at equal segments and the space between the bungie cords forms a cube.

Now grab several bungie cords close to each other and pull them close and tie them together. The bungie segments closest to those segments get warped and stretched out of position while bungie segments further away are relatively unaffected.

In space the spots where the bungies are tied together represent massive objects like stars and planets. And the amount of warping/stretching represents the gravity well around that object.