Shandeh is right; so far you have made the cuff and instep, or top of the bootie. While I have not made this specific pattern, I have made cuffed booties using a pattern that is almost identical to this one.
To sew the back seam means to fold the cuff you made in half, joining the edges to make the top part of the bootie. Once you have the back of the cuff sewed together, that seam should basically be the center back. This is where you will be starting to make the remainder of the foot part, going from that center join, around one side of the instep, across the front of the instep, and back again around the other side.
Hope this helps!