A mitered border meets in the corners at 45 degree angles as opposed to a simple border.
In the first quilt below, I used one fabric (blue fabric) for the outside quilt border. I used a small red and white striped fabric for the small inner border.
In the second quilt, I used many of the scraps left over from the quilt top for my border. I used the blue fabric for the inner border and a red fabric for the small “piping” border. (See the closeup photo at the bottom.)