A seam line is the visible join where two halves of a part meet — down the side of a head, along a forearm, across a rifle. Molds split parts in half; assembly puts the split back on display.
Some seams are designed to read as panel lines and can stay. The rest get removed: cement the halves so the joint melts together, let it cure fully, then sand the ridge flush. On snap-fit kits without glue, builders either live with seams or upgrade to cement for the pieces that show.
See it in context in our sanding & surface prep guides, or explore the tools hands-on in The Bench.