We are very well suited to imagine three dimensions because our world as we sense it is a three-dimensional space. You may remember learning about Cartesian coordinates in three dimensions, where any point in 3D space is represented by a coordinate (x, y, z).
Now let’s imagine that instead of each point representing a point in space, each point represents an entire universe. Imagine parallel universes moving through this three dimensional “space”. Imagine that universes can “split” into two parallel universes and also that two parallel universes can join together. We will come back to this idea later. For the rest of the day think of our universe as a giant spaceship traveling through a space filled with other universes.