This means that the elements of a matrix consist of several parts. For example, if the element type of a matrix is Point3d, it is 3-channel, because Point3d is a 3-dimensional point (x, y, z), Size2f is 2-channel, and uchar is single-channel.

For example, when an image is represented by a matrix, uchar is a grayscale image with 1 channels. The color image Vec3b is 3-channel and contains BGR components.