Before we talk about extra dimensions, let us first review what dimensions are. A dimension is a way of measuring something along a certain direction. For example, a line has one dimension, because it only has length. A square has two dimensions, because it has length and width. A cube has three dimensions, because it has length, width, and height. A point has zero dimensions, because it has no size at all.
We can also think of dimensions as independent axes that we can use to locate any point in space. For example, we can use the x-axis, the y-axis, and the z-axis to describe the position of any point in a three-dimensional space. Each axis is perpendicular to the other two, and each point has a unique set of coordinates (x, y, z) that tells us how far it is from the origin (0, 0, 0) along each axis.