An antenna tuner, also known as a transmatch, is a device for correcting an impedance mismatch between a transmitter or receiver and an antenna.
The Tuner Fallacy
It is often believed that a tuner actually "tunes" a non-resonant antenna, tricking it into behaving as though it were resonant. This is not the case, as a simple experiment will show.
Suppose that we set up a transceiver system like the one below.
Suppose that the tuner is set to direct or bypass operation, connecting the two SWR meters directly together. If the transmitter is keyed, both SWR meters will indicate the same level of SWR. Assume that the non-resonant antenna has SWR at both meters of 10:1.
Now if the tuner is taken out of bypass mode and its inductors and capacitors adjusted until it produces a perfect match between transceiver and antenna, the SWR meter at left will be indicating 1:1, but the SWR meter at right will still be indicating 10:1. In other words, our "antenna" tuner is actually tuning the transmitter to the antenna, not the antenna to the transmitter. We might better call it a transmitter tuner or transceiver tuner! There will always be feedline losses on the line between the tuner and the antenna. To minimise these keep this connection as short as possible and use the best cable you can afford!
The most common amateur antenna tuners are the many models made by MFJ. Other manufacturers also produce tuners. The assembly of a tuner is so simple that many hams build their own from scratch using homebrew techniques. Simple schematic: