A mitre or miter joint is a beveled location or joint where two pieces meet. The two pieces to be joined are often beveled at 45 or 90 degrees. The problem with this kind of joint is that it is weak. And a small error or a gap between the two of materials will result into a disconnected joint.
Even if you were able to create a good miter joint, a good amount of force can cause the joint to have a gap. Of course, there are other joints like finger joints and mortise and tenon, but multiple factors (such as time constraints) may lead you to use a miter joint instead.
Usually, the fix for a miter joint gap is to create a spline. The spline is a thin wafer of material that goes between the two materials to be joined. It reinforces the glue and strengthens the joint.
Image source: Larry Satchwell
Move on to the Next Page video: Closing the gap in a miter joint.