Seam allowance, often shortened to SA, is the extra fabric at the seam that allows you to sew a garment together.
If, instead of sewing, you could magically join seams together, there would be no need for seam allowance.
Since we can’t do magic, we need a bit of seam allowance so that the minor fraying of the fabric will not undo the seam.
Seam allowance can differ. 1cm is common for some patterns, but in the US 5/8” or 15mm is standard. Make sure to always check the pattern for the default SA.
Extra SA can be required for specific finished (like french seams) or for hemming.
Picture by Kristin Roach