If seamlines are a garment's building blocks then seam allowance is a seam's building block- a seam (or seamline) cannot physically exist without seam allowance.
In basic terms, seam allowance is the distance from the seamline to the fabric (or pattern) edge. When looking at a pattern, you can find the exact seam allowance width along each edge by measuring from the seamline to the pattern edge. The marked seamline depicts the seam's stitch line.
Seam allowance represents the intake necessary to sew and finish a seam or fabric edge. The width of the seam allowance should be chosen base don type of fabric, where the seam (or edge) is located on the garment, and what type of finishing techniques will be appleid to the finished seam. Most regular seams have a seam allowance of 1/2" or 3/8" as this can accomodate most seam finishing techniques. In some instances, seam allowance can be wider 3/4" or 1" to allow for the garment to be altered into a larger size later on, or to ensure a durable connection within thick, bulky fabric seams.
Unless otherwise specified, once the seam is stitched, seam allowance layers are usually ironed open. Ding so allows the seam to lay smooth and flat by spreading out seam allowance bulk evenly on both sides of the seamline.