A creeper allows someone to lay down on it move under a car that has been lifted safely. The creeper has wheels that allow for easy movement, but tend to be the weak point as far as weight limits are concerned. Some creepers fold for storage, seating position, head position, or a combination of these.
Creepers tend to be more popular with DIY mechanics as opposed to those that work at a shop with vehicle lifts that can raise the vehicle much higher than a jack and jack stands can.
Considerations When Choosing a Creeper
Wheel Size
Large wheels are easier to move over floor imperfections and debris like zip ties, but they tend to raise the creeper up, which means the vehicle being worked under must be lifted higher.