The word jet has several meanings:

1. A powerful stream of liquid or gas shooting out under pressure. When you turn on a garden hose and put your thumb partly over the opening, you create a jet of water that sprays much farther. Fire hoses use jets of water to fight fires from a distance. Inside a jet engine, burning fuel creates powerful jets of hot gas that push an airplane forward. You might see jets of steam rising from a teakettle or jets of water shooting up from a fountain.
2. A jet airplane, which is an aircraft powered by jet engines. These engines work by sucking in air, mixing it with fuel, burning it, and shooting out a powerful jet of hot exhaust that pushes the plane forward. The first jet planes flew in the 1940s and revolutionized air travel by flying much faster and higher than propeller planes. Today, almost all large passenger planes are jets. When someone says they're jetting off to Paris, they mean they're flying there (and feeling pretty fancy about it).
3. A hard, glossy black stone used in jewelry. Jet is actually fossilized wood that's been compressed underground for millions of years. Victorian-era mourners often wore jet jewelry. When something is described as jet-black, it means it's as dark and shiny as this stone.

As a verb, jet means to travel somewhere quickly, especially by airplane.