Babies usually learn to crawl before they develop walking skills.
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 he/she cannot walk because of being an infant or due to disability or being wounded or sick.
In very low places (caves, under a table, in a mine, etc.). Sometimes underground miners need to crawl long distances during their work.