Crawl is a command that sets your method of locomotion to crawling.
crawl You get yourself into position on the ground, preparing to slowly crawl like you've never crawled before.
crawl stop You stop crawling around. Your tortured arms thank you for this. You stand up.
Crawling allows you to get around when you have broken legs without hurting yourself further. It also lets you get through certain low exits.
Crawling is slower than walking. You have to wait a certain amount of time after crawling into a room before being able to crawl again, and it costs 25 adventuring gp per crawl. You can "crawl fast" which costs 50 adventuring gp per crawl.
Also, your dexterity is reduced by three while you are crawling.