A cocktail jigger is a small measuring device used by bartenders to accurately measure out the ingredients for a cocktail. It is typically made of stainless steel and has two sides, one with a larger measurement (usually 1.5 ounces) and one with a smaller measurement (usually 1 ounce). The jigger is used to measure out the spirits, liqueurs, and other ingredients that go into a cocktail, allowing the bartender to create a consistent and balanced drink every time. It's a standard tool in any bar, and is considered essential for making a perfect cocktail.