All that means is that the binding fabric is to be cut on the bias rather than straight-of-grain. The only reasons to cut binding on the bias for a table runner are if: (1) the runner has curves, say at the corners, and/or (2) the binding is a striped fabric, to get the stripes running at an angle.
To cut a bias binding, you typically fold the fabric on the bias and then cut strips, making sure your ruler is 90 degrees to the fold (to prevent bends).
Straight-of-grain binding is cut at a 90 degree angle to the other straight-of-grain (crosswise or lengthwise; there are 2 straight-of-grains in fabric). Cutting the fabric at 135 degrees means cutting 135 degrees from the straight-of-grain, which means you are cutting on the true bias (which is exactly between 90 degrees and 180 degrees).
Hope this is understandable!