foundation piecing is when you are sewing your pieces to another layer which serves as the foundation. In my photo you can see the white poly that I am stitching mine to. You can also use paper, muslin or whatever you want. If you use paper, you tear it off afterwards. If you use any type of fabric, it remains a permanent part of your block. A foundation provides stability when you are using lots of small or narrow pieces. I didn't need one in this block but just wanted to try the technique.