The E6B is a "flight computer" designed to perform common aviation calculations during flight planning, or in-flight.

The E6B is a "flight computer" designed to perform common aviation calculations, such as "How much fuel will be burned in 3 hours in cruise flight?" or "How long will it take to travel 248 nautical miles at a speed of 132 knots?", conversions such as quantity of fuel (liters or gallons) to weight (kilograms or pounds), and calculating the effects of wind on cruise flight.

The traditional E6B is a mechanical slide rule, as shown below, however electronic E6B calculators are available, and E6B functionality is often incorporated into other aviation applications like weight & balance calculators or flight planning software.
Mechanical E6B

For an example of how to use a mechanical E6B see this YouTube video.