Hi Debbie, when you cut a square diagonally you get a triangle. The long side of this triangle is a bias edge-It stretches and when you sew that side you have to be careful not to stretch it as it will distort the HST you are trying to create.
In fabric there is 1. straight grain- runs the length of the fabric ( up and down) 2. crosswise is selvedge to selvedge (side to side) and bias is corner to corner of the fabric.
If you pull on a piece of fabric that is on the straight grain it won't stretch. when you pull on a piece of crosswise fabric it will give a little (stretch). Now take a piece of fabric and pull on it corner to diagonal corner you will see that it stretches alot-that is the bias of the fabric- so when you're working on a block that has alot of bias edges you have to be careful not to stretch the fabric while sewing or pressing -because if you do you will end up with WONKY blocks.