I am new to quilting and sewing. I am working on a project that calls for 1 3/8 inch single fold binding. I have only worked with 2 1/2 inch binding that gets pressed in half. What is the difference?

The double fold will have twice the amount of fabric on the edge of the quilt, creating a more durable, longer lasting binding.
Cut yourself a small 1 3/8 strip and fold it to see the difference.
Some older patterns might be written that way. But there shouldn't be any reason why you couldn't substitute the double fold instead.

ooc, was it to be used on the outside of the quilt, or in joining blocks together?

what's the project? what you mentioned is store bought binding

If your project is a wall hanging, table topper or something like that I would say it will be OK. For a quilt, no way. The binding is usually the first thing to wear on a quilt.

This is a kit and is a 15 by 15 wall hanging or table topper with wool appliqué I can't do my normal binding as I don't have enough fabric. So. I don't fold in half. Just sew on? What do I do about raw edge on back?

You would fold under a 1/4". This is usually pressed before applying it to the quilt.

Ah....now I get it! Thanks so much!

Lay your binding strip wrong side up. Now bring in the raw edge from one side about 1/4" and press.

Now lay the other raw edge of the binding on the front of your quilt and sew the binding on as you normally would. Turn it to the back and hand/machine sew it down.

I would use a slightly larger than 1/4" , closer to 3/8" when sewing it to the quilt sandwich.