Normally sounds are created by an object creating vibrations in the air around it. These vibrations reach our ears and we perceive them as sounds. Drums, guitars and flutes are all examples of acoustic instruments that make sounds.
In a guitar, for example, the player plucks the string and it vibrates at its resonant frequency. The player can change the length of the string by pressing the string against the fret board at various positions, allowing him or her to play different notes. The tone produced by the string is then modified by bouncing around inside the body of the guitar, giving it a sound that is unique to guitars.
A synthesizer is an electronic device that creates these vibrations in the form of an electrical signal. The vibrations can be sped up or slowed down, giving different notes. They can be manipulated and modified in many fun and interesting ways that can’t be done acoustically.
At some point, the electrical signal must be amplified and fed through a speaker. This converts the signal into vibrations in the air that we can hear.