To crawl is to move slowly across the floor on your hands and knees. Before they learn to walk, most babies crawl.
You might crawl around looking for a lost earring, or watch a spider crawl across your ceiling. The slow, creeping movement itself is a crawl, too: "Traffic was moving at a crawl." When your dog has fleas, he crawls with them, and if something gives you a creepy, prickling sensation, it makes your skin crawl. Crawl probably comes from the Old Norse krafla, "to claw one's way," which shares a root with crab.