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A transformer changes the level of AC signals, or the impedances of circuits. They consist of a minimum of two coils, the primary and the secondary, wound on the same core. An ideal transformer has no losses, it merely steps a voltage up or down in proportion to the turns ratio between the primary and the secondary. This is useful in converting the voltage from a wall outlet, typically 120 or 240 volts, into a higher voltage for the tube plate supply, typically 400V or more, and a lower voltage for the tube filament, typically 6.3 or 12.6V. The transformer will also "reflect back" to the primary the impedance which is connected to the secondary, in proportion to the square of the turns ratio. That is, if you have a 20:1 transformer with a 16 ohm impedance connected to the secondary, it will "look like" a 6.4K ohm impedance on the primary side. This is useful in matching the plate of a tube, which is very high impedance, typically on the order of several thousand ohms, to a speaker, which is very low impedance, typically on the order of 4, 8, or 16 ohms.