A stitch is a pattern of forcing thread up through fabric then back down. The pattern can be repeated. Stitches can be decorative (cf. Embroidery) for trimming and ornamenting clothing or accessories, or functional for holding two or more pieces of fabric together to make clothing, garb, or accessories.
A stitch is also slang for a cramp: a "stitch in your side" means a painful shortness of breath.