Sew describes stitching something together. If you sew up the holes in your socks, you use thread and a needle to close them.
The word sew comes from the Old English word siwian, to stitch. You can sew a patch on a pair of jeans, sew a dress, or sew up a hole in your grandmother's old quilt. A figurative meaning of sew, as in the phrase to sew something up, is to secure something or bring it to a happy conclusion, like sewing up the plot in the last chapter of a book. Don't confuse sew with sow, to plant or set in motion.