An afterburner is a component present on some jet engines, mostly those used on military supersonic aircraft. Simply put, an afterburner works by injecting jet fuel into the exhaust air and igniting it. The ignition of the jet fuel increases pressure within the exhaust of the engine, thus increasing thrust. When you see flames coming out the back of a jet engine like on military aircraft, this is because they have engaged their afterburner.
The F-15 has two engines which under normal power produce 14,590 pounds of thrust. The F-15 can not reach its maximum speed of Mach 2.5 using normal thrust. When the afterburners are in use the F-15's engines each produce 23,770 pounds of thrust. The afterburners can be used as long as there is fuel. The objective of an afterburner is to increase the thrust by about a factor of 1.5 to 2.0 for short periods.
Jet engine thrust is governed by the general principle of mass flow rate. Thrust depends on two things: the velocity of the exhaust gas and the mass of that gas. The afterburner increases thrust primarily by accelerating the exhaust gas to a higher velocity. While the mass of the fuel added to the exhaust does contribute to an increase in thrust, this effect is small compared to the increase in exhaust velocity. The purpose is to provide an increase in thrust, usually for supersonic flight, takeoff, and combat situations.
Afterburning significantly increases thrust without the weight of an additional engine, but at the cost of very high fuel consumption and decreased fuel efficiency, limiting its practical use to short bursts. An afterburner uses the unburned portion of the air passing through the engine it is attached to, to add an enormous amount of extra thrust. Most afterburners will burn substantially more fuel than the jet engines they serve, with fuel consumption tripling compared to normal operation.