Seams are a garment's building blocks. Seamlines are technically a flat representation of the seam on each portion of the garment, If you take a look at your sewing patterns, the seamlines should be clearly marked at a distance for each pattern each, either with a dotted line or a solid line. In dressmaking, seamlines are also referred to as stitch lines as they depict the exact stitch placement for each seam or finished edge.
Another way to look at samlines is from the perspective of seam allowance. In other words, a seamline is located at exactly the seam allowance width from the edge inward, If you are a sewing beginner, it may be a good idea to transfer you seamlines from your patterns to the corresponding fabric pieces, especially for the first couple of projects, This is done using fabric-safe technique such as tailor's chalk, fabric pencil or tracing paper.
On the face side, two seamlines that are stitched together create a seam (or finished edge, depending on seamline location). Seams cannot exist without seam allowance regardless of how wide or narrow this seam allowance is.
In the sewing process, transferred seamlines serve as convenient stitch lines especially if you are a sewing beginner and not quite comfortable relying solely on the machine's seam allowance guides. As you stitch directly in the marked seamline, the fabric's raw edge should align perfectly with the appropriate seam allowance guide designated for the seam at hand. Even if you use only the marked seamlines to stitch your seams, you should still keep an eye on the machine's seam allowance guides to ensure an even, precise application.