Before the first dimension, there is a dot, a singularity, to add another dot and connect them is to create the first dimension, a line. To add another line and connect them is to create the second dimension, a plane. To add another plane and connect them is to create the third dimension, a space, an image of a cube will be the easiest to understand. Now to add another space and connect them is to create the fourth dimension, but you must understand that this is not time because the whole point is that these are spatial dimensions, existing simultaneously within a single moment.
To imagine the fourth dimension, it is easiest to take a step back and imagine yourself as a 2D being. First, understand that as a 3D beings, we perceive the world in 2D, we see everything as a flat image, with only highlight and shadow to imply that most objects extend forward and backward within a 3rd plane. So as a 2D being, you see in 1D. What does this mean?
Imagine you are represented as a circle on a page and in front of you is an other circle. So you, from your eye, as a circle on a plane, can only see your fellow circle as a line. You cannot look down upon the circle from above because you are not a 3D being. So to understand that the circle is a circle you must move around it, noticing where the lines of its form start and stop. You also, as a 2D being cannot see inside of the circle, because that would require looking at it from above or below, within a 3rd dimension. A 3D being can see all the sides of the 2D circle at once. From this analogy, one can infer that 4D being could see all the sides of a 3D sphere at once. Imagine being able to see the back, front, bottom, top, inside and outside of a globe simultaneously, without having to move yourself or the object.
This is the fourth dimension.