For humans it usually means moving on knees and hands, with support from the toes. Except for using a vehicle or being carried by an animal, it is the main alternative to walking and running. Crawling is used mainly when a person is not capable of walking because of being an infant or due to a disability or being wounded or sick. It is also applied in very low places (a cave, under a table, etc.), when searching for something on the ground, and for fun, including playing a submissive role in BDSM.
Figuratively, crawling means moving slowly.
See also web crawler.