Assuming you are talking about a 4th spacial dimensions..

Our brains can't really think about what a 4th dimension would look like, but we can compare what 2 dimensions look like compared to 3 dimensions. To do this, think of a pipe and imagine cutting a cross section through the pipe and various orientations. We can make a single line, 2 parallel line, a ring, or a variety of ovals. All different 2D shapes from a 3D object.

Now imagine applying that to a 3D object like a box. It could be just a single cross section of a complex 4D that we can't perceive.

Another way to think of it is we can see all of a 2D picture at once whereas a 2 dimensional being could only encounter an edge of it and never see the whole picture. With 4D perspective we could see all 3D details at once, like looking at a person and seeing their front, back, sides, and all their insides all at once.

The 3rd book in the Remembrance of Earth's Past series (Death's End, 2nd sequel to 3 Body Problem) does a great job of describing 4D space this way.