Crawling as a type of movement is normally only undertaken when a creature has either been knocked Prone, or chosen to drop Prone voluntarily.
To move while prone, you must crawl or use magic such as teleportation. Every foot of movement while crawling costs 1 extra foot. Crawling 1 foot in difficult terrain, therefore, costs 3 feet of movement.
For example, a bard with 30 feet of movement speed would only move 15 feet on their turn if they decided to drop prone in an attempt to crawl past a window they fear they might have been spotted from. Should that same bard try crawling past that same window through a tangle of bushes however, they would instead move only 10 feet.
Standing up from prone, thus ending your need to use Crawling as a movement type, costs half of your character's base Speed.