A leader is a small scrap of fabric. You sew a seam across it, line up your real seam, sew it and then sew another scrap - the ender. Why bother? It saves thread and it keeps the machine from eating small corners like triangles - some zigzag machines like to eat bigger corners, too. I rarely use them because I can strip piece almost anything. I have my next seam ready before I stop sewing my last one, so my good fabric is my 'leader and ender'.
My 13 year old GD made her first quilt using leaders and enders. She reused the same few scraps over and over . She found it helped her start and finish each seam straighter and she had one less thing to do because she didn't have to remember to hold the threads as she started sewing. I like not having any extra threads to trim when I'm done when I use them.
Bonnie Hunter's idea is great, but it's not the only way to use leaders and enders.