A serger (or overlocker) trims fabric edges, stitches them together, and finishes them in one fast step, creating durable, professional-looking seams, especially for stretchy knit fabrics, by encasing raw edges in thread to prevent fraying. It's a specialized tool that complements a sewing machine, handling tasks like rolled hems, decorative finishes, and quick garment construction where stretch and clean finishes are key, but it doesn't replace a regular machine for features like zippers or buttonholes.