Parachute
From Kerbal Space Program Wiki
Parachutes are parts that when deployed slows down the speed of a spaceship in an atmosphere by creating drag.
There are 2 different parachutes, Mk16 Parachute and Mk16-XL Parachute.
Usage
Parachutes are commonly used in Kerbin re-entry to slow the spaceship to a landing. They can also be used in Duna to help the landing engines, as they are very inefficient when used alone there.
Don't forget that parachute's efficiency depends of atmospheric density (they require a minimum air pressure of 0.01), so it won't work or even deploy in a vacuum.