A musical synthesizer is an instrument that uses electricity to make musical sounds. Very often synthesizers have a keyboard like that of a piano. When playing a piano keyboard, sounds are made by hitting soft hammers against strings pulled very tight. When playing a synthesizer keyboard, sounds are made by turning electrical oscillators on and off. Since "synthesizer" is such a big word, the word is usually shortened to "synth".
An oscillator is something that "vibrates," or repeats the same pattern. The pendulum of a clock, for example, is a very slow oscillator. A piano string "oscillates" when struck by a hammer.
Electric oscillators might be made using transistors. They turn electricity into electrical patterns, or signals, that repeat over and over. When different keys are pressed, different notes are heard. This is done by making the signal from the oscillator repeat at different rates.

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